First thing to do is check the current draw of your AC. There should be a manufacturers tag giving the model number and electrical requirements. Second, the electric water heater is probably drawing nearly all the current availale, when it is heating water. Cycling between water heater and AC may be necessary to prevent overloading the circuit. Third, although most home wall sockets are wired to 20 amp breakers, the wall sockets themselves are only rated at 15 amps. True 20 amp sockets will usually have a pin that is verticle to and connected to the + . ( two parallel pins for + and neutral, with a round ground pin below and a pin verticle to the + pin.)
