I can't imagine a transmission heating issue with the boat in neutral - the heat in the transmission comes from the fact that there's 300 HP coming in from the engine, and only 299 horsepower leaving the transmission. Some amount of the power gets taken up by friction, etc., and turns into heat. No friction, no heat.
But, if you dump the water in before the cooler, like SP suggests, there won't be a problem.
BTW, PO means "Previous Owner" - the person who gets blamed for every bad thing that's ever happened to a MasterCraft.
I don't think the garden hose attachment is going to do anything for winterizing. Some use the 5-gallon bucket to suck RV antifreeze into the engine for winterizing, but that's about it.
1998 Maristar 200VRS