How to shut off until plumber replace the leaking pipe

where does it seem to be leaking from?

Water on top of gas heater can be caused by poor venting, or inadequate combustion.
Make sure water is not coming up from under tank top, indicating tank leak requiring tank replacement.
Have a plumber look at problem to make sure home is not filling with deadly, odorless CO gas.
Inspect pipes that they are not corroded and covered with condensate.
Check that roof cap is in place.
Let heater fire and burn for a few moments so the fire is going good, and then light match just outside the draft hood on top of heater. Blow out match and see if smoke draws immediately into vent and upward.
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html
http://waterheatertimer.org/Leaking-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Water on top of gas heater can be caused by poor venting, or inadequate combustion.
Make sure water is not coming up from under tank top, indicating tank leak requiring tank replacement.
Have a plumber look at problem to make sure home is not filling with deadly, odorless CO gas.
Inspect pipes that they are not corroded and covered with condensate.
Check that roof cap is in place.
Let heater fire and burn for a few moments so the fire is going good, and then light match just outside the draft hood on top of heater. Blow out match and see if smoke draws immediately into vent and upward.
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html
http://waterheatertimer.org/Leaking-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Very simple - call a Plumber. He will fix your plumbing problem, and any foreseen potential problems related to your water leak. He may also suggest replacing your 6 year old water heater. Of course ..... for a professional cost.
However, depending on your time, skills, proper tools, patience and luck, it's easy enough to perform yourself:
1. Switch off the water heater Breakers (220v) in your service panel.
2. Turn off the water supply to the water heater.
3. Attach a water hose to the drain faucet. ( a good time to flush sediment -calcium deposits)
4. Open a hot water faucet to vent (above the level of the water heater drain - kitchen sink)
5. Remove the suspected leak components. (a union connection is a good place to start)
6. Install New pipes -3 wrap all male threaded ends with teflon Tap. Tighten securely - using 2 wrenches.
NOTE: Before refilling - you can turn on the water heater supply valve for a few minutes, to flush out any lingering calcium debris.
7. Shut off drain valve - remove hose. Shut off venting hot water faucet.
8. Turn on water supply valve to fill water heater.
9. Open a hot water faucet to relieve air pressure while filling - preferably the bath tub faucet, it doesn't have a screen that might clog.
10. When water flows without air - shut off tub faucet.
11. Look for leaks.
12. Switch the water heater Breakers on.
Note: 50 gallons should take an hour or more to heat up. Turn on a hot water valve momentarily within this heat up time to relieve thermal expansion. ( the PT valve on your water heater will perform this task if pressures/temperature exceed new water heater tolerances - it's just a precaution to introduce these fluctuating pressures on your fragile 6 year old unit)

These are just basic instructions - assuming you fill in the common sense and work safe steps.
Good Luck - The Skill will only come after trying.

I am uncertain from your description exactly where water is coming from.

I want you to protect yourself first.

Read following page to help identify source of leak:
http://waterheatertimer.org/Leaking-water-heater.html

If your vent pipe is spitting water back on top of water heater, then the problem is not the water heater. Problem could be poor venting.
Water heater is putting odorless CO gas into home, and will kill you. Give you headaches, make you feel sick until you go outside and get fresh air.

Call a plumber please if flue pipe is problem.

Here's how to check if flue pipe is drafting properly:
When water heater burner is off, light a match, blow match flame out, put match under vent hood on top of water heater.
Does smoke from match immediately draft up flue pipe?
If smoke does not draft up strong, then water heater is putting CO gas into home.

The vent on all gas appliances has to be working properly.
Look for missing top of vent pipe located on roof.
If cover is missing, a bird might built a nest on end of pipe, or leaves have fallen into pipe which is a fire hazard.

If flue pipe goes into chimney, you should call a plumber.
A plumber is cheaper than potential health problems with improper flue.

Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, expert speaks with you over phone while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

If I understand correctly, your Bock gas water heater vent is causing water to leak down into chimney.
This is a very serious concern

Water leaking from water heater flue indicates improper venting.
The hot air gasses from water heater combustion contain water vapor, but the gas should vent out of house before condensate forms on pipes.
http://waterheatertimer.org/Water-at-bottom-of-water-heater.html#flue

The problem might be caused by a power vented furnace that pushes vent air from furnace down into flue pipe for water heater. As a result, the water heater doesn't vent properly.
There could also be a clog.
Light a match under water heater draft hood and see if smoke from match travels instantly upward.

There are two major concerns.
1) If vent is not working, it means that odorless CO gas is entering home instead of venting outdoors.
CO carbon monoxide is deadly poison.
http://en.wikipedia.org/wiki/CO_poisoning
This is a very serious concern
You can buy CO detector after vent is repaired.

2) Water condensate will eventually rust away vent pipes because of acidic content of the combustion vapor. It can be expensive to replace pipes, and pipes with holes pose risk of CO poisoning.

My suggestion is call a qualified plumber ASAP and explain what you observe.
Plumber will want to inspect the vent pipes, and may want to extend water heater vent pipe up further inside chimney so furnace is not pushing air back into water heater vent.
Plumber may also find blockage somewhere in the vent stack or chimney.
Also post on a professional water heater forum like the Tank to double check anything the plumber says to you. These are the pros.
http://www.thetankatwaterheaterrescue.com/forums/forum3/

If you have a leaking water heater there is a serious problem.
http://waterheatertimer.org/Water-heater-leaking.html
Page shows how to limp along on a leaking water heater if your water heater needs replacement.

One of three things is causing the water:
1) Loose water connection on top of tank that is dripping down inside tank >> check pipes on top of tank.

2) The vent is clogged or not working correctly causing condensation inside flue pipe.
Condensation water is dripping back down inside tank.
Problem means possible CO poisoning if gas is not venting.
Look at vent pipe to see if joints are corroded.
Use long fireplace match > light match and put into draft hood at top of tank and see if smoke goes up.
Put hand at draft hood opening when water heater turns on to see if vent gas enters home.
Do same test when furnace turns on to see if furnace vent is pushing back into water heater.
Does water heater vent inside same pipe as furnace? You may need to set up separate vent for water heater if furnace gasses are pushing back into home.
Check furnace vent for obstructions.
Look at vent stack top of home to see if hood is in place.
Check if bird nest is built inside warm vent pipe.
Improper flue is top priority for getting fixed so vent gasses properly escape home.
You may have to call a plumber.
Professional plumbing forum:
http://www.thetankatwaterheaterrescue.com/forums/forum3/

3) Your water heater is rusted out.
A rusted tank has to be replaced.
It cannot be repaired.
However, before replacing tank, you do not want a tank failure where tank ruptures and water runs inside home until water is shut off.
This will cause major damage.
Protect yourself first.
Turn off water valve above tank.
Move gas knob to 'pilot' so burner is not heating water.
Open tub spout on hot side to relieve pressure on tank so it will not rupture.

Now you're protected.
Link above shows how to limp along on failed tank until you can replace heater.

You need a new gas water heater from Lowes or Home Depot
Look at label on side of you tank for gallons and btu's of your water heater.
Measure height to flue pipe so you get same height water heater, so pipes don't have to be changed.
You need a pipe wrench and teflon tape which will be wrapped on each pipe thread when pipes are reassembled.

Step1) Drain water heater using valve at bottom of tank.
http://waterheatertimer.org/Water-heater-will-not-drain.html

2) Turn off gas valve going into heater.
When gas valve is perpendicular to gas line, then gas is off.
Dissemble gas line to water heater.

3) Disconnect flue pipe and support flue pipe with wire so it doesn't not fall down. Flue pipe joints should be held together at each joint with 3 screws.

4) Turn off water. Shut off valve is on COLD side.
Disconnect water pipes.

5) Move old water heater out of way.

6) Set new water heater in place. Put water heater in drip pan.

7) Connect water pipes >> cold goes to side marked cold.
Hot to hot.
Buy new flexible water connector pipes at home center for easy installation of water pipes.

8) Connect gas line exactly as it was connected before.
Home center sells flexible gas line connector for easy installation of gas line.
To test for gas leak after installation, mix liquid soap with water and put around gas line joints to see if there are bubbles.
Natural gas has less pressure than water lines, so gas pipes are easy to seal with ordinary wrench.

9) install TP pipe that runs to floor. Use old one off old heater.

10) Fill tank completely with water until water is coming out of tub spout.

11) Turn on gas, and light pilot as shown in manual.
New gas heater sometimes have small water leak when first fired up, so ignore a bit of water.

12) Save water heater manual like it is gold, and read everything in manual.

It sounds like you need to replace it, Actual cost can very widrly depending on your location but I would say $350-400 is probably close. Plus installation unless you can do it yourself. Hope this helps. Thanks

What is the leaking pvc pipe connected to? The T & P valve on the side (or top) of the tank? Or is it an incoming cold or outgoing hot water line? If you have just lit the water heater, the T & P valve may drip, but only for a little while and it will stop. The T & P (temperature and pressure relief) valve may be bad and need replaced. Turn off the incoming cold water valve and turn the gas valve to off position. Then drain the water heater using the T & P valve, it will stop draining when the water level is just below it. Remove the T&P valve and replace with new one. Use thread sealing tape on threads. Or call a plumber if not sure.

If it is the pipe leading in or out, you can repair it yourself with a torch and solder, after draining the pipes and turning off the gas.
If it is the tank itself, you will need to replace it. The tanks cannot be reasonably repaired, so replacement is your only option. Legally, you must call a certified gas fitter (or plumber with a gas ticket) to handle the removal and replacement.
I suggest shopping around yourself for a heater as prices vary widely, and plumbers generally charge more than a home improvement store like Lowe's or Home Depot. Have the heater on hand when the plumber arrives and it will be a very quick job.