The 20-year-old Four Winns Liberator 211 with Chevy 5.7L Mercruiser I/O
left us stranded when it wouldn't crank after anchoring in the ICW.

My initial thoughts were battery or connections, or (yikes) a
hydrostatic lock since we had been stopped in some chop for a while. So
after towing back home I took out the sparkplugs and she would crank.
However I could not find any water coming out the sparkplug ports after
some cranking. I did compression tests one cylinder at a time with all
the sparkplugs removed, and it would hardly crank whichever cylinder was
compressing, as I tried each of the eight in turn.

Up to this point I hadn't considered a bad starter, since they usually
just quit altogether, but I decided to test it anyway. I put an ammeter
on the cable and found it was drawing 400 amps (!). I took the starter
off and ran it in a vise and found the same draw while it ran freely.
The engine manual said 60 or 90 amps was about right for a free=running
starter.

I disassembled the starter and found two of the four permanent magnets
inside had come unbonded from the case, and shifted around in position,
and were rubbing on the rotor. I glued them back in place with epoxy,
reassembled everything, and now it cranks and starts like new.

In retrospect, I should have caught that the engine had always cranked
weakly during the month I've had it, despite having a freshly charged
battery and good connections. The 2-gauge battery cables always got
warm when starting. I should have known better than to overlook and
ignore such warnings.

I don't think this starter was the original. It has a cast aluminum
body and zinc chromate housing, with a sticker that says MAS over an
anchor. Apparently nearly all GM marine engines use the same starter,
which appears to match the Sierra 18-5913 type with permanent magnets.
Sierra 18-5905 and 18-5910 were also listed as replacements, but I
couldn't find what the differences were.