Those of you that have larger boats with central ac. You can have your hvac tech install a High pressure hi temp gas to water heat exchanger on the discharge (high side) of the ac compressor/s R-22 and 410A both have pretty high temps that are well above 100* This would NOT serve as on demand. The logic against this idea is.. "well if I'm at dock using Electric for the AC then why not just use the electric WH. But, there are always bragging rights I guess
Now, if your boat has an on board generator the reduced load by pre cooling the refrigerant will lower the ac's amp draw and also reduce or eliminate the amps required for the electric element in the WH.
Jus a few words from your friendly neighborhood hvac lady
ps: this would work best on 24,000 btu ac or larger. smaller ones would not be worth the trouble of breaking into the refrigerant circuits.