2000 Mercury Sport jet 175 xr2 question

I have a 2000 Regal 1700 Xr2 Jet boat with the mercury 175 sport jet. When I took the boat out of storage, I hooked the hose to it and the engine started right up. Went down to the lake put the boat in the water, it ran for about 3 min and died out. would not restart. I took the boat to a local mechanic, they said it was the stator. They installed the stator and when I picked the boat up and put it in the water, same thing. Runs for 3 min and dies, does not restart. I took it back to the mechanic. They tested it and could not find anything. It ran for over an hour on the trailer. I changed plugs, fuel filter and ran a can of sea foam thru. Still runs on the trailer perfectly and dies after 3 min in the water. It restarts as soon you pull it out of the water. Please help !!!!