When you tow anything it is considered in that "Heavy/off road" use category. I does lead to higher temps and more wear on the drive train and even the motor. More frequent fluid changes are recommended and you should be nearing your 2nd as the first one is 60k and then 30 there on after if i remember correct from the manual.
This probably isnt the cause of the boat and just a seal issue as stated above unless there is a crack or some other damage that is hidden, which is really unlikely.
As far as the diff, i really cant give you any idea, there isnt fluid in the shaft and the input pinion is sealed from the universal so id guess water but not sure there. maybe your parked at a decline and fluid is running out the input pinion seal onto the universal but with out looking i cant tell.
Also your tranny fluid should be a redish heavy fluid, not clear?