Capt. Matt wrote:
works for air boats...

Noisy. Very noisy. On an airboat, no-one cares about the fan, it's
lost in the hurricane. On a boat, you don't want to start the engine
for a little assist at the dock and suddenly have to put up with
yelling over a cooling fan.

Many boaters crave tranquility on the water, not hi speed thrills.

Besides, a rad and fan weigh more than a piece of hose and a
waterpump you already need, especially since, if you don't want hot
exhaust pipes in the bedroom, you want additional cooling capacity
to cool the pipes.

It's cheaper and less hassle to use raw water or even a keel cooler.

With a rad, you also have a blast of hot air to contend with.