Re: Oil in exhaust?
In a boat, it is important to understand that the load on your engine is tied to your rpm and speed through the water. The load on the propeller determines the load on the engine and the engine will happily run at any rpm unless the load is greater than the power that it produces at that rpm (this is the problem with being overpropped). If the boat feels slower at a given rpm, it is not an indicator of the engine being down on power, it is an indicator of more drag or a less efficient propeller. In your case, it sounds like you were aground so that may well have put more load on the engine. It may well be that you were getting close to the edge of your available power for the given rpm so the engine was fueling heavily which can lead to black smoke.
Gray smoke indicates poor combustion in the cylinders. Common causes are incorrect injectors (this is only a problem for the guys building performance diesels) and a cold engine. From your post, it sounds quite plausible that your engine was cold so that may be the explanation. Black smoke is from overfueling and you don't have enough air to fully burn the fuel. This occurs when you really load down the engine. On older naturally aspirated diesels, this is controlled through a fuel plate normally which controls how much fuel is available at any rpm, on turbocharged ones there is an air fuel control which supplements the fuel plate to control fuel based on turbo boost. If you have a common rail engine, then it is likely that you have a MAF sensor that controls this. This is all a bit off the subject but it controls your smoke producing ability to meet emissions regulations and keep your engine together.
If your boat goes back to motoring at normal speed for a given rpm and you don't get anymore smoke, I wouldn't worry about it. If you motor too slowly, then you should check the prop. If you motor normally and get smoke, then look at the color and try to determine when you get it and someone should be able to help you.