1) If leak is simple, then put garden hose cap over end of drain valve.
Pick up hose cap in garden department at home center or hardware store
2) Or install a garden hose and run to a drain until valve can be replaced.
The hot water will not last as long for showers until valve is fixed
3) If drain valve is cracked, it needs to be replaced.
If you are a DIY adventurer, then replace drain valve.
Since drain valve is plastic, expect to use every tool you can borrow when the plastic valve breaks off.
4) Call a plumber who will prefer to replace with new heater.
5) If you want adventure, then replace heater yourself and save $400
installation. Here in Houston 2011, electric heater costs 220 and gas
330 at local box store. Offer 10% less.
Open following link for gas installation, and use basic information for replacing electric.
Good luck and Upvote the help.
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on timer or any do-it-yourself project.
Fixya is always less expensive than a service call.