Having just gone through a breakdown where I had to rule out hydrostatic
lock (turned out to be a half-bad starter), I'm thinking about the risk and
how to avoid it on my 5.7 Mercruiser I/O. I have never seen a disassembled
exhaust system to know what the plumbing geometry looks like inside. It's
not clear to me how this works, because you obviously have raw water going
through the exhaust all the time in a running engine.

Can you get hydrostatic lock from a following sea or chop splashing up your
I/O exhaust ports *when the engine is running*? Or does the running engine
prevent that? How much of a wash does it take if your engine is cut? I
have 4-inch stainless ports, with a slightly uphill slope through the
rubber tubes to 6-inch risers. The bottom of the port is only about 8
inches above the waterline. Seems like a little too much trim astern and
some wave action, and you'd have water quite a bit over those ports.

Do people use some kind of plugs or stopper on the exhaust when anchored or
just drifting? I have the usual rubber flaps just inside the ports on the
stern, but I'm thinking of something like a foam stopper that you'd shove
into the exhaust port. If you forgot to remove it before starting, it
would just pop out, captured on a lanyard.