Most boat conversions have a second water pump that actually pulls the water from the lake. It typically uses a rubber impeller inside this pump to suck the water into the engine and split it to the manifolds and traditional water pump. These things go bad pretty quickly ( annually).

Typical boats in NC run a 160* thermostat. If you have a temp guage that may give you a clue if you are getting enough cooling. It is typical to see a "hot" engine if you run a boat hard, then shut if off, there is a LOT of residual heat. These engines work a lot harder than a car on the highway. Try to restart it in 3-4 minutes and your temp may be over 200*. But should cool back down again. A way to lessen this is to idle for a few minutes after a hard run.

Lots of good ideas above. Take out your thermostat for a while and see what happens.