I use the Merc high perf as well. And you are right to start changing it in the fall. That way it does not sit with contaminated oil over the winter.

I make sure I run my boat for a few minutes after changeing the fluids to flush clean lube through before winter. Engine oil becomes acidic from combustion deposits and can pit the bearings if left sitting for long periods. I don't think that is a problem with the drive, just moisture to worry about there.

It doesn't sound like you have much water intrusion but keep your eye on it. You can pull the lower plug after the boat has sat for a day and just let a little dribble out. If there is water it will tend to settle at the bottom so you will see it right away. As long as you only remove the bottom plug the oil just trickles out so you don't have to change it all just to check for water.