After about an hour the breaker trips on a single element Rheem water heater. 2Meg ohm to ground from the element terminals: 700K between element termianals. theromstat connections to ground infinate. (can hear normal hissing noise when ac applied) Replace element or breaker?

Breaker keeps tripping means short in wire or heat element
Shut power off . use ohm meter and go from ground to each wire on elements and if you get continuity. there is short pull wire off element check wires to ground not continuity bad element.
still shorted follow all wires may have bare wire shorted against cabinet Or by new heater

Turn power off. Open covers and inspect for burned parts.
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#trip-breaker

Test elements. Marathon takes specific element, and does not use generic element from hardware store.
http://waterheatertimer.org/How-to-test-water-heater-element.html

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

It could be element shorted to ground through steel tank, so don't touch tank when breaker is ON.
Or you have undersized 12 gauge wire and 20 amp breaker and the heat has loosened breaker on the busbar. Breaker trips when heat exceeds breaker limit. Jiggle breaker and listen for crackling sound.

To restore hot water now:
Turn off circuit breaker, and open top cover then Push in reset button on top of heater to restore functionality. Put back insulation and factory cover.
To fix water heater: test and replace bad element.
http://waterheatertimer.org/How-to-test-water-heater-element.html
If problems continue
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#trip-breaker
http://waterheatertimer.org/Troubleshoot-household-electricity.html

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

You have dual element 240Volt tank-type electric water heater.
You want to change wiring from non-simultaneous to simultaneous.
Copy following link for exact internal wiring diagram, and external wiring change: Remember the wire from breaker box will not support both elements without tripping breaker, so you will need additional 30 amp breaker and 10 gauge wire, or single 50 amp breaker and 6 gauge wire.
http://waterheatertimer.org/How-to-wire-water-heater-thermostats.html#simultaneous
http://waterheatertimer.org/Color-codewire2.html
http://waterheatertimer.org/Figure-Volts-Amps-Watts-for-water-heater.html

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

Problem could be a shorted element. Disconnect the wires from the breaker and try to reset it, remember to flip the switch in the fully off position before moving it to the on position. If the breaker doesn't trip then the problem is in the heater itself. If you have access to an Ohmmeter, with the power turned off check the continuity of the elements across the screw terminals and from the terminals to ground. There should not be a reading to ground and a high resistance across the terminal screws

find the leak first, then dry out the wiring and thermostat. extremely hot water indicates one of the elements has gone bad and is returning to ground. If you have a ohmmeter, with the power off disconnect the wires to the elements and check from ground to the terminals. If there is deflection on the ohm scale, you have a grounded element. I suggest you also relpace the high limit cutout as well, it's the part of the thermostat with the red button.

Remove upper and lower access panels
in upper access, use voltage tester to test voltage of on the uppermost terminals above the red reset button. should be ~220V between terminals (110 to ground)
If not: breaker off or bad,
If 220V proceed:
Test the two terminals immediately below this reset button should be 220V
If not: reset tripped or bad
If 220V present, proceed.
Turn off breaker to heater.

Use ohm meter or continuity tester.
test resistance across upper element terminals -should be 10 to15 ohms
check between each terminal and ground -should be open circuit .
If not replace element
Repeat for lower element.

test across lower element thermostat should be no resistance when stat turned up so it clicks on.
(To test that lower stat turns off, allow tank to heat up completely before conducting this test) Turn sta temp down so it clicks off then test across stat terminals -should be open circuit

The upper thermostat is a 3 way switch
The simplest way to test it is to turn the power back on turn the upper stat all the way up so it clicks on and use voltage tester across the UPPER element terminals, should be ~220V
Now turn the upper stat down so it clicks off- check upper element should be 0 V between upper element terminals -(Caution ! terminals will still be live 110V to ground)
The upper stat has now sent power to lower stat.
Turn lower stat up so it clicks on, check Voltage across lower element -should be ~220V turn lower stat down so clicks off, should be 0 V across lower element terminals.

NOTE If tank is completely cold the upper stat will not switch off and wont send power to lower stat.
If lower tank is cold lower stat may not click off even when turned down.
normal stat setting is 125 to 130
The hotter you you set the stat, the faster your tank corrodes.

Hi , when a breaker trips immediately, that can indicate a problem in the circuit. But the circuit stays on for a bit , then trips, so there is a problem with the heater. Could be that a thermoastat or heating element is shorted out. You can replace these items yourself, I would check the heat element first. Or there may be some corosion residue around the heater terminals.

Have you checked to make sure that the elements are good by checking the ohms. Be sure that the power is off and the wires are disconnected from elements and then also be sure to check the ohms across the tank to make sure there is not a dead short in the element reading through the tank. If both elements are good then I suggest replacing both thermos then they will both be new and should work for you . Thermos can be problematic when trying to test them sometimes they won't always give you correct readings. Make sure you have the breaker off whenever you mess with a waterheater, and make sure you install the thermos correctly with the right wires going where they should.

If you don't have current, 0v from terminal to terminal, then you don't have voltage to the elements. Replace the thermostat but check that you have 220v to the thermostat. It is possible that your breakers to the unit are either bad or tripped.