FWIW, when I first got my boat, the trans started slipping one day, mainly in turns and starting out. Noticed I was a bit low on fluid. Filled it up, that cured it, then happened again a few weeks later. Took it to the shop (before I got into wrenching), they found that the seal around where the shift linkage goes into the trans had a small leak. Got it fixed, had them put new clutch pack in (existing ones were about half worn out), no problems after that. Just another spot to check for leaking in case anyone else has this issue.
1989 ProStar 190 with "Safe T Top", 351 Ford, GT 40 heads painted Ford Blue, Elec. Ign, 1:1, 720 hours, 4 blade prop, PP 6.5, removable platform brackets, custom color from factory, hot water shower, Boat Buddy & LED lights on trailer
2010 D3 Z7, 2 @ 32 off, 34 mph so far