To cool the exhaust, seawater is pumped into the exhaust system. It is done at a piece called a seawater injection elbow (since it is usually bent)--the odd-looking stainless steel piece in the picture of the back of the engine.

When I got the boat, there was a tiny leak in the seawater injection elbow, which really wasn't a problem...just put a small amount of water in the boat, but eventually it would rust out enough to put a lot of water and fumes in the boat, so I wanted to replace it at some point. It is hard to import parts into Brazil, and people are used to making more than ordering, so they made this one for me (and also a spare one, because the hot exhaust gases will eventually corrode this one also).