Since the boat was running fine before you changed the power valve I would double check the valve. The valve might be defective or leaking around the gasket because it was not tightened properly. Use a little petroleum jelly and hold it upside down when you tighten it to make sure the gasket stays centered.
Super easy to test the valve, screw in both your idle air screws (count how many turns so you can put them back where they were) and if the engine starts to stumble and die the valve is good. If it keeps running the valve is bad.