It’s a through hull port for many things, although what you’re observing sounds like a bilge pump.
Boats leak. They also (unless they’re quite large) use holding tanks for black water, but just dump sink or bathing water into the bilge, which is then pumped out the side.
The back of boats do this too, if it’s using a heat exchange water system to cool the engine. It sucks in water, cools the engine, then dumps it back outside.
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