Expanding a bit on the previous advice:
If you have little blowby, then the overheating event could be limited to a blown head gasket, warped head or cracked exhaust manifold, sort of in that order. Even in the worst case, all three could be repaired in the boat without pulling the engine, much cheaper than the other alternatives you are considering.
So, check blowby as Ski suggests and go from there.
David
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