Hate to see someone not get a fast reply; there are quite a # of possible causes. But the most likely cuplrit it is running too lean and needs to be richened up. Things like a airleak in the gas lines, carb seal, head gasket can also cause an engine to be too lean. Another possibility is too big of a prop. Others more experienced than me will chime in if you tell us which boat/hull, engine, prop you are running.

todd