We have an electric water heater. The basement flooded with 2 1/2 feet of water. Now when I take a shower the water goes cold in 3 to 5 minutes. What could be wrong?
Replace the thing, it's shot. The lower thermostat if not the lower element's toast, and several other parts could have failed as well, in addition for the potential for waterlogged insulation causing...issues, especially if some foam-eating mold moves into it.
Oh, and your tank's likely to have started rusting from the outside in if the insulation has water in it, too.
If the water heater is more than 5 years old it may be prudent to replace it to avoid problems in the future. Conversely, if you'd rather not purchase a new water heater you can first replace the lower thermostat for about $20 as opposed to several hundred dollars.
Shut the power to the water heater off at the breaker panel. To access the lower thermostat open the lower panel cover. Inside you will find a plastic cover protecting the thermostat. After removing the cover check for power with a voltage tester to be sure no live wires exist.
Next make a wire diagram of the wires and how they attach to the lower thermostat terminals. Loosen each terminal screw to free the wires. Depending on the model the thermostat may be removed by pushing on a tab or pushing a clip. It is usually held in place by a metal clasp or clip. The hard part is done!
At the hardware store match the replacement thermostat by the "Wattage" from the old one. Also check it has the same wire terminals.
Installation is in the reverse order. If you successfully installed the new thermostat the water should be heated in about 1 hour depending on the tank size and the thermostat setting.