Jim wrote in :

I saw this parts breakdown a couple days ago, but I'm still confused.
The thermostat shown in this picture is not on my engine. If it was
there, it's been removed. There is one in the top of the thermostat
housing that sure looks like it was cast with it in there (which seems
like a very odd thing to do).

If this is anything like my Volvo Penta 4.3 (and I believe it is since the
Volvo Penta was derived from the OMC Cobra) there is a retaining ring, its
just very hard to see. I had the same thought as you, that the thermostat
looked like it was cast in place. But I also knew that this was unlikely,
so I searched for what I figured must be there, a retaining ring. Couldn't
find it, so I took the whole piece to my friendly marine shop. When I
asked the counter guy how to get the thermostat out, he said "there's a
retaining ring." He took a tool with a point akin to a dentist's pick and
inserted into a groove just next to the thermostat which I hadn't seen
before. Probably obscured by corrision/debris. The ring is rubber and he
slipped the "pick" under and behind it and lifted the ring out in about 5
seconds. I felt pretty stupid but grateful. The thermostat pulled right out
and the new one dropped right in. The ring was in perfect shape and the
shop didn't even want to sell me a new one, so we popped it back in and was
done. Bolted the housing back on the engine and was in business.