Yeah, there are boats that run in 10° weather. And, as they head to the north, to get snow crabs, they have to keep breaking ice off the deck, so the weight does not build up, and capsize them.
These motors have an open cooling sys, BUT also have a thermostat.
That means that the ocean water goes into a place in the exhaust, where it is heated to a certain temp, then it is dumped back into the ocean. From this source the cooling water for the motor is taken, (It is like a mini reservoir in the manifold) which is probably around 100°, but I don't know. I learned this by taking apart a boat motor a long time ago. Curiosity got me to looking. And, the motor was locked up with water in it. I got it running!
Anyway, Open cooling, with a thermostat is the answer.
Apparently, Sea doo does not employ this?
Nate