trouble shooting cooling system

Finally finished restoration of my 67 barracuda ss. Remanufactured 318 (original block was frozen), new stringers, floor, carpet, wetsand and polished gelcoat. It looks great and sounds even better. However the engine heats up very quickly when on the water. The boat has two water pickups (one has two hose hookups and the other has one)instead of one, and uses a dual impeller pump to pull the water out of the lake. This engine has no circulation pump like a car. It just uses the dual impeller pump. Is the hose leading from the thermostat housing a return? Also could the problem be my thermostat or impellers? I would like to talk to someone who has this boat or same engine (318 poly). Anyone's advice would be greatly appreciated.

Thanks Tim...Yes it is heating upu sitting still. I made it about 100 yards out into the lake and realized it was getting hot, so i shut it off for about 10-15 minutes and then putted back to the landing. Hope I did not hurt anything.

I agree I would change the impellor. When it gets hot sitting still, that is the sign that it's getting ready to go bad. I would also check to make sure you don't have any blockages inside the hoses.