Raw water flow problem!
I agree with bubb2 - sounds like a raw water flow problem. As bubb2 said, check the raw water system from the suction thru-hull to the exhaust outlet. Also, there is normally an anti-siphon loop in the raw water supply to the wet exhaust. This loop normally has a vacuum breaker at the very top - pull and check that - exhaust soot sometimes plugs these up. Also check the elbow where the raw water is physically injected into the exhaust (follow the hose). This is another spot for exhaust soot and rust to plug the raw water flow. Good luck!