Hello. My 355 tranny slips and revs out only after running for 15 minutes. 2 questions.

1. It was suggested it may be overheating. Could this be the cause? My next run I’ll put a laser temp huge on it and compare with the other. What should the temp be?
2. If not, tranny removal. Can this be done in the water and what are the instructions to do so? I’m concerned about the prop shaft and have been told several different things, including my boat might sink. It has drip less seals and the 355 has long shafts supported by a bearing support below the boat.

I’d like to know for sure and not just suggestions please. Also one of the Hull valves is stuck open and should be changed on my next haul out.