I just learned this over the weekend as I discovered my problem. Those with the stated motor, mine is a year 200 225 EFI, on a jack plate may have this same issue. The lower unit is cooled by the water and being on a jack plate may not allow it to cool sufficiently. The result is the oil heats up, expands and pushed the water pump housing up into the impellar housing. This ruins the seal on the water pump and impellar housings and allows water to get into your lower unit. It also ruins your impellar as the upward pressure pushing on the plate below the impellar ends up "eating up" the impellar from the bottom, drive shaft center outward. If you have such a motor, you may want to check it this year before you hit the water. Merc makes a non OEM water pump to solve this problem from happening again. Merc has this listed as a service "issue" for the motor i mentioned. Thought I would pass this along.