Our engine is only a 43 HP, so actually draws only about 20 HP while running at normal load.

This makes me wonder whether there is enough heat dumped into the cooling water to be able to recover a useful amount.

I plan to include a circulating pump which will pull off the line leaving the engine and feed back into it.
Thus, if the pump is not running, virtually no water will flow through the heater.

Having the pump should avoid issues on hose length (about 30 ft round trip) and the level, (as long as I bleed any trapped air out at the high spot)

Any other advice on installing?