max camirand wrote:
If air cooling didn't work out, you could remove the radiator and
instead pump the coolant through a coil of copper pipe running through
a sea chest (small seawater-flooded compartment built inside on the
inside surface of the hull). Kind of like a keel cooler, but inside.
The advantage over cooling with raw seawater is that you don't get
corrosion inside your engine, both from the seawater and from the
galvanic action it creates between the dissimilar metals in the engine.
Of course, it's slightly more complicated.

Why bother with all that complication. Just go on E-Bay and search for
"Heat Exchanger" and you will find a plethora of all sizes and shapes.
Then add a raw water pump on a bracket, some plumbing and replace the
radiator with the heat exchanger.