[parishht]

I know you are on a lake with fresh water, but mine was in salt water and the saltwater ate away at the gaskets. Check your compression inthe cyliders or if you know someone with a scope, have them look at your pistons.
My engine had slow water intrussion and eventually burned 1,3 and 6. I kept losing power over a three year period, started and seamed to run fine on land with hose, but put it in water and it would start and stall right out when put in gear, turned out to be running on three cylindars.
This could be worst case scenario for you.