motor overheating

My friend did the same thing with his Wellcraft, same motor. He likes to dredge the bottom too. He ended up packing his riser with crushed shells and sand, some small pebbles too. The heat exchanger was piping hot, hoses and all which told us at least it was working properly, but it wasn't exchanging the heat with the saltwater coming in. We pulled the outgoing hose off the riser at the back where it meets up near the rubber boot as the exhaust comes out. It connects the exchanger with the back of the riser. Stuck a garden hose in there, real quick started the engine and turned on the water. The pressure built up immediately because there was crap packed in there. He pulled the riser off and you could see the stuff inside the exhaust part where the water meets to exit the engine. He worked at it with a small hanger a lot of water and I think a real thin knife. He cleaned it out, put it back together with new gaskets and such. It works fine now. He had cleaned out a beach's worth of sand and stuff from his heat exchanger but did not realize that what was sucked in would also try to be forced out. It jammed up at the exhaust end of the riser. You'll see what I mean when you get the riser off. You'll see the inside collar that exhaust comes out and around that is the passage that water exits from. That small space was jammed with grit.