I note that you are experiencing a reduced exhaust output at power, which may be the problem. You may have a clogged exhaust manifold, or line, which creates back pressure at power and heats up the water because of the lack of exhaust flow. Check your exhaust riser muffler, and associate hoses. Also see if your exhaust manifold is a lot hotter than the rest of the engine. I would imagine it should not be too much hotter under normal circumstances. Jim