While winterizing boats last fall, I noticed a problem with the water intakes on many boats.
Engines may overheat (which could cause expensive repairs) due to Zebra Mussels blocking the engine cooling water intakes.
Keeping the trim in the up position when not in use may help but it's recommended (especially if the boat sits for a period of time) that you check to see that the water intake holes are clear. They are located on each side of the outdrive - see attached image. If there is a build up of mussels, they can be easily removed with a piece of wood and knife to clear the holes - be careful, they're sharp.
The good news is, when the boat is removed from the water, the mussels will die and should fall off.