Today, I took my boat out for the first time this year. I got the boat in the water, and it started up right away with no problem. I let it run in neutral with the throttle lever up just a little bit for approximately 10 minutes to warm up. No problems. It ran fine the whole time.
Then, I went out on the water. I slipped it into gear and took off out of the channel and into open water for about a half hour at various speeds with no problems. Then, when I slowed down, preparing to stop, the engine stalled. I was not even in neutral when it stalled. I attempted to start the engine again several times. Sometimes it would just turn over and not start. Sometimes it would start for a few seconds and then die. Sometimes, after starting, I would raise the throttle lever all the way up, but the engine would not rev higher. Sometimes, with the throttle lever all the way up, the engine would rev high for a few seconds and then slowly rev down and stall. Sometimes I would raise the throttle lever all the way up and the engine would not start revving higher until a few seconds after I lifted the lever. Then, with the lever still up, it would rev down and stall again. No matter what I did, I could never keep the engine running for more than 5 or 10 seconds.
After about 15 minutes of not being able to keep the engine running, and in an attempt to get back to the dock, I was then trying to start the engine up with the throttle lever up to get it to rev high, and then I'd quickly drop the throttle lever down and shift into gear quickly to get moving. The first few times I tried this, it stalled. Finally, I got lucky and the boat started moving and the engine kept running long enough to get back to the dock.
As soon as I slowed down near the dock, however, the engine stalled again. I could not start it up again and keep it running.
I'm sure you'll get advise from real mechanics here - but from my experience, this sounds like a dirty carb issue. Giving credit to the gurus on this forum, you should probably start by doing a compression test to make sure there are no major issues (although the fact that you got up on a plane and ran it OK probably argues against that). A spark test next will tell you if you are getting good spark. I've never had powerpack issues on any of my motors, so I can't help you if you have spark issues. If these tests go well, then you probably have some gunk in your carbs. That's always been my experience when I have hot-start issues. Lots of help on this forum for cleaning carbs. Make sure you re-assemble looking at the diagrams.