1992 Mariah overheat then got water in cylinders

[JSPulliam21]

Yesterday my overheat alarm went off on our way back home. I stopped and shut the boat off of course, but when I did there was a gurgling sucking sound for a bit I never heard before. During the teardown of my raw water pump only to see that the impeller was fine I pulled the thermostat to see that the it looked like an archaeological discovery. I assume it stuck closed and caused my issue, but I will verify today with a pot of boiling water. Could be an air sucking issue too, and I will rectify that as well. However, the starter would move the engine but wouldn't turn it over. I knew better than to keep trying and pulled the spark plugs before turning it over again. It blew water out of at least 4 holes which confirmed my suspicion. My question is how did that happen!? What failed in method or system that caused my boat to suck water up the exhaust into the cylinders? How do I prevent this from ever happening again? 351 PCM carb'd engine btw.

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