Re: Both stater bolts sheered off 3.0
The engine may have hydrolocked while it was sitting. If the starter was healthy and the battery strong then when it tried to turn over the engine and a cylinder hit the hydrolock condition something had to give.
Water can get into a cylinder thru an open exhaust valve thru rusty manifolds or risers. Also, if an engine is shut off suddenly and the boat is still moving then exhaust water can end up travelling back down into the exhaust passage. Also, if an engine runs on after it is shut off then that can cause what is called reversion, and water can get sucked back down the exhaust. These are just some of the ways an engine can ingest water into the top of a cylinder. When the engine tries to compress the water it can't and that is a hydrolock.
This is just one potential cause of your event.
To check this remove the spark plugs and turn the engine over by hand and see if water is pumped out any of those holes.