Ok so we took the baja out for the first time a few weekends ago and the boat was running hot, I slowed down to let it cool down and it shut off and wouldn't restart, I know it over heated at that point. Went to start it and starter went out, so I got a new starter and it wouldn't even turn over, it was hydro locked. I pulled number 6 plug and water poured out... It actually fired up with that plug out but I only let it run a second...

I bought the boat used last year and have been battling issues, the manifolds and risers are supposed to be new, they do look it.... So what could have caused that cylinder to fill up like that? It's just a stock 454 mag mpi.
I'm rebuilding it now but still want to. Know what caused it and I had to leave the boat and motor with a shop and I'm not getting much support out of them so I'm looking for suggestions I don't want to put it back together and repeat this.

Is the exhaust sloped towards the back or is it straight? Are you sure the manifolds are new. Have you pressure checked them?

If it overheated bad enough it probably lifted the head from the gasket. I would be concerned about the rings. Check the deck surface & the heads before you put it back together. Been there done that. Randy

ive got the shop testing the manifolds, or atleast trying to get them too. The exhaust is standard bravo exhaust with captains call
Randy my thought was that also, that it lifted a head just like a normal motor would. i had the block decked and bored and the heads milled. putting a complete scat forged stroker bottom end in it so all those parts are new.
just hoping it isnt the exhaust i cant really swing that right now..

The motor will probably run lean if the cubes are increased much unless the ECU is re flashed.