Please check exhaust manifolds, I have a same problem with 6.0 LY6 Indmar engine in my X35!
First I change several starters before I found broken exhaust manifold! When you swich off the engine, Water flow from manifold thry exhaust valve in to cylinder and cose hydro stroke!
This summer I had a same problem with other manifold, this time it cose total damage to engine, boat is now at my dealer, they try to get a new engine from Indmar!
Hope this would help,