Water in fuel?
It happened to me recently, white smoke (water vapor) came out of the exhaust, and I found that there was a substantial amount of water in my primary fuel filter. After I changed the filter, the boat "burned off" the remaining water in the line and produced white smoke, which is water vapor. Check you filters for water and consider installing a water separator if your boat doesn't already have one. There are also fuel additives which can help your engine pass the water through the system harmlessly. See my recent post in this forum about water in the fuel and the replies which I received.
