No, an isolator is the wrong choice. Isolators divert current
to the LOWEST load so it will always be sending power to the water heater
even if your batteries
need a charge.
on the alternator will cut the current
down as the batteries
reach full charge so you don't normally want to be wasting fuel
to heat water at such low efficiency. If you NEED extra water heating
look into a heat exchanger
The wind generator
output is free (more or less) and although I can't name a source I understand there are regulators that already have a "dump" output to connect to a resistive load like a water heater. This will only divert energy to the heater AFTER the battery has been charged.