230 is pretty hot for a marine engine, especially considering that the thermostat is probably between 120 and 160. Could be a blown head gasket which caused it to overheat and/or cause rust in the cylinders which has hurt ring seal. Engines that sit for long periods of time (like most boat engines) can have sticky valves. Test for sticky valves with a vacuum gauge. It will show up as wildly changing vacuum numbers if its an intake valve. Pulling the valve cover will also help you diagnose sticky valves.
My guess is that it is a leaking head gasket that has led to other problems. Without the anti-corrosion benefit of anti-freeze, plus the added minerals and things in lake water (or even worse, salt water) can make short work of head gaskets and water passages if not flushed really well before storage.