CorrosionX at the end of the day and after a dunking. Spray a bunch in at the end of the day, wrap a towel around the lower unit and let it drain until the next event (this assumes your adapter has holes that line up with the motor vent holes).
I tend to spray all my motors when new just for a little added protection.
It is a good idea in my opinion to remove motor off lower unit assembly when done racing for the week to let the water that gets trapped in the pto endbell to dry out. It will save that bearing, you should re oil outboard motor bearings before any storage.
I will also add that after purchasing a couple motors used that were in inboard applications, i found rust on the stator. I think its a good practice inboard or outboard to at least corrosion x before the first use. With an outboard wd40 or cor x everytime. Inboards if you know you got it wet. It doesnt take much to get rust buildup with these motors! Mike