I was wondering if anyone had some advice to determine the failure of my Yamaha 80 4 stroke. It would be really appreciated.

Basically, I went out on the water (bungs left out - yes I know ha), and could not get on the plane, realised taking on the water. Headed back to shore. Boat took on a lot of water, the back of boat pretty low in the water, but I don't think the motor really got submerged at all (although it could have a bit, I wasn't really paying much attention).

In getting the boat back to the ramp, it was low in the back end, and really churning up a lot of rocks and so on.

Anyway got it back home (30 minutes later). Went to put the motor down, but not power. The 30 amp fuse in the motor had blown. Fixed that.

Now started it. The motor just didn't idle right. Something weird happening. It sounded like it wasn't all firing on an all cylinders, or a tapping noise (or a heavy tappet noise), so much you pretty much had to turn it off, sounded bad.

Anyway, drained fuel (even though has a fuel filter that takes out any water etc). Cleaned out the boat. Still no joy, same. My father in law suggested something in motor - muck, so took out spark plugs, and turned it over. Put it all back. Much better - sounded fine.

Took for a test run on the water. It made the tappet noise for a bit but then cleared. Also, made a squeaky noise (like something need oil) but that went as well. Anyway took it for a good run for 20 minutes. All good and fine. Sounded fine.

Went on the flat and thought I'd give it full throttle. BANG. She blew.

Apparently the cause: after the mechanic inspected the motor (and two assessors) the motor failure is a result of a hydraulic lock from oil blow by in the number 4 cylinder. This is most likely a result of the number 4 piston oil ring becoming seized in the ring Lan.

Apparently, there was no evidence of water ingress that would cause this to happen.

This was a fairly new motor (6 years old) and well kept and maintained by Yahama Dealer. Just had it's service a month back.

My confusion is, what was the cause of this major failure - it would seem to much "chance" that a mechanical failure happened at the same time of partial submersion of the boat incident?

Anyway insurance won't cover it because they claim it is just the fault of the motor. I beg to differ and don't understand.

Any ideas what people think the cause of this would be?