You getting water in the engine.
Replace the risers gaskets and the problem should go away. If not, you may have to replace the manifolds. Maybe they are crack.
I would go first a replace the gaskets. That is the cheapest. $20-30.
This season you use the boat and was OK?
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-2001 Maxum 2100 SR2 Sport -1997 Regal 258 Commodore (Sold)