I have a tohatsu md50, 2003 3 cyclinder. Motor was running fine for approx 3 hrs then blew a head gasket. Possible cause was a blocked thermostat with a bit of plastic lodged in there.
Gutted thermostat, skimmed head approx 6thou and skimmed deck / black approx 10 thou. Only enough to flatten the surface areas.
Motor has little to no corrosion. No pitted edges on flat surfaces.
Replaced two head gaskets now, torqued properly to spec stamped on head. 18 ft pound.
New bolts installed.

Again tested today with new gasket and still plugs coming out milky. Dropped in the water for quick run and no power due to water in cyclinders. Definitely leaking, not just moisture in the block.

The only thing I can think of now is that the gaskets used are non genuine and maybe not sealing as good as original gaskets?
Base gasket has been replaced but power head pulled on off 3 times, gasket still looks ok nice and thick with no tears but maybe this is not sealing and somehow ingress water through the base plate gasket?
I don't know what else to do, my boat is out of action until I can get this water leak fixed. The head looked good with no signs of cracks, although I have not pressure tested the head. Could the exhaust ports cooling plate be leaking somehow?
I was under the impression that only a head gasket would allow water in the cyclinders not a base gasket or side cover exhaust plate gasket.

Any help or suggestions would be most appreciated, I can not afford to keep buying head gaskets and now I don't even know for sure what it is. The only option I thought was to replace the head gasket with genuine and try again.