All of our boats have a bit of combustion deposits on the transom after some time and this is normal.  It's not normal to visually see anything in the exhaust gasses under normal engine loads other than a bit of water vapor.  Blue, grey, and white smoke can often be misidentified.  Blue is lubricating oil being burned, white/grey is either unburned fuel or water in the exhaust.  Don't mistake normal cooling water vaporization for something more serious.  The easiest way to tell if anything is a problem is to monitor your fluid levels:

Lubricating oil consumption in a healthy engine is almost zero.  On my engine with 600 hours, I don't have any measurable oil consumption between changes (100 hours).  If you find yourself having to add lubricating oil on a regular basis, then this is where blue smoke is coming from.

Coolant consumption in a healthy engine should be low.  You'll loose a bit over time as coolant is moved to/from the overflow reservoir, evaporation from the reservoir, hoses expanding, etc.  Perhaps a few ounces a month.  Significant and persistent loss of coolant along with white/grey smoke is indicative of a breakdown between the cooling circuit and the intake/combustion circuit.

Finally, abnormal white/grey smoke in the absence of the above fluid loss symptoms is indicative of unburnt fuel.  Unburnt fuel is either injector timing, bad injector, or low compression.

I don't use any fuel additives.  The most common use of additives are to prevent "jelling" in fuel when it becomes very cold.  Hope this helps.