You might get and use an infrared temperature gauge. They're not that expensive any more. Reading on the exhaust
manifold should be roughly equivalent to the boat gauge. Not being able to stand on the engine cover sounds too hot to me.
Is your sending unit in the thermostat housing or on the front of the engine. Mine used to be on the front of the engine and I moved it to the mounting point supplied on the side of the thermostat housing. It seems like that would be a more accurate reading point.