featherweight doesn't like water

[grandpa]

If I had hair I'd pull it out.
After messing around with my Featherweight for a while, it will now start and run quite smoothly in a water tub as long as the prop is not attached. I have also run the engine very briefly out of water with the prop on and feel that it would run all day like this.
It will not run with the prop in water.
I thought it might be the back pressure of being in a tub, but no, it won't even start in the river when strapped onto a dinghy.
Although the piston/rings/bore look absolutely fine, I can't think of anything else other than it just hasn't got enough oomph left to turn the prop under pressure.
I'd be grateful for any ideas before asking John to get me a new piston/rings and/or whatever else it needs.
cheers,
Grandpa