excessive oil in exhaust? 40 hp 2 str. Merc.
don't know if you pull the boat out of the water, but if you do, the unburned oil drips out and is usually very dark, almost black
as a precaution, I would also take a look at the oil in the lower unit to be sure it isn't milky or low
Your motor needs 50:1 at WOT, but could likely get by at 75:1 at idle. When you mix your fuel at 50:1 and don't get above idle speeds all of the oil does not have a chance to burn off, and you see what you are describing...
Every once in a while open her up to WOT.