If you are seeing white smoke, is it water vapour? do you have a good flow of water out your exhaust? Perhaps your water pump is not working properly.
The black smoke could just be old build up in your exhaust hose. I am planning on replacing mine this year and cleaning out the exhaust riser. Any restrictions in these will reduce the engine power.
Good luck my last boat had the same engine and I loved it. Always started even in below freezing temperatures.