[zellerj]

I think this is a water lift muffler. Hot exhaust gases are mixed with the cooling water exiting the manifold, and that mixing cools the exhaust and quiets the noise. The exhaust pressure then pushes the mixture out the transom.

If this muffler fills with water, say from excessive cranking without starting with the seacock open, or a big following wave pushes sea water backwards through the exhaust hose, it can lead to water getting into the cylinders via the exhaust manifold. That is why you should not crank the engine for a long time without it starting with the seacock open. If the engine does not start right up, it is best to close the seacock until it starts.