Some information that may assist in your diagnosis of how your engine is running - by the colour of it's exhaust:
Black smoke - Is an indication of unburned fuel or the engine is highly loaded. If the boat is moving at hulll speed and the engine has enough power to try to make it go faster, black smoke will result. Back off the throttle a few hundred RPM's until the black smoke disappears or reduces to grey smoke.
Black smoke with an oil sheen and soot on the water may indicate plugged, worn or dirty injectors. Running the engine hard for 15 minutes or so may clean up the exhaust. If after this "blowing out" period the black smoke continues, service of the injectors may be necessary.
Grey smoke - Is an indication that the engine is running well. All diesel engines produce some grey smoke and may deposit a sooty film on the transom of the boat. If a lot of soot is present, see "black smoke".
White smoke - Normally indicates steam. Check the engine temperature and the volume of exhaust water to insure the engine is running at the proper temperature. In colder weather, white smoke is normal when the engine is running at operating temp.
Hope this helps someone.