Well your typical electric water heaters draw 4500Watts of electrical power. Basic rule of thumb is 2HP required for every 1KW of electric power produced for sustainable electric power generation. So you need at least 9HP to power an electric water heater.
Induction generators are fascinating, but when setup for standalone power, they are really lacking in one key area, and that is voltage regulation. They require some form of excitation(as do all generators) to output power. There are 2 ways to do this. the first is to wire them to an electrical source, and spin them above their rated RPM. This requires no regulation as the induction motor will sync to the line voltage and frequency, and the torque provided by the engine will feed current back down the line. This is the old school way of selling power back to the utility if you have a source of torque such as hydro power to spin the motor. The second way is to use capacitance across the lines. The capacitance stores a little energy from one cycle and feeds it back to excite the next one. The ammount of capacitance determines the output voltage. But since the capacitance is fixed, change the load and the voltage changes and requires a different capacitance to get the proper voltage. This is said to be a pretty efficient way to make power, but it is VERY difficult to control.
What RPM is your engine? For a few hundred you could get a ST-3 head and belt drive it off your engine to get 1800 generator RPM(required RPM for 60HZ frequency). With that engine you could sustain a little over 2.5KW of electric load which will probably power all the necessities such as a furnace, reefer and all the lights and TV. IT of course is still not big enough for a 240V 4500W water heater. There are ways to power a water heater with 120V that require far less electric power, but of course have a much slower water heat time...