Mercruizer 4.3 L one side exhaust manifold over heating

[wssautobody]

I had my boat out today and as i was running about three quarter throttle for maybe `15 min, the rh exhaust manifold got so hot it melted the rubber boots on the exhaust manifolds. i checked that there was water flowing out the bottom of both sides and the engine block. so i don't think it is the impeller. But i am not sure. Why would it be just one side. Any help is grateful. It is on a 1986 Sea Sprite with a 4.3 L mercruiser.