Thermostat

This was leaking. With the kind help of other boaters, I now know that it's our engine's thermostat housing. They also told me how to get it off, and how to fix the leaks. If the big brass bit is welded onto the engine body, it's (allegedly) quite a simple task to redo the joint again. If it's not, then all that needs doing is to replace a couple of gaskets, and make sure the top screws are torqued correctly when tightened up. Then I can slap some more red paint on it. job done.

Looked more serious than it actually was.