I’m voting it’s just condensation. It happens to our boat when ever we put it on Lake Michigan in May and June. Water is cold as ****. Any other lake or further into the season I never see it.

I’ve seen it that bad from condensation after a short jaunt in frigid water and oil temps never came up. That was with an oil thermostat as well. I pressure tested the cooling system to verify it was not leaking internally as I found it odd that both motors looked the same as yours do. Cleaned out engines and put T stats back in, went for long cruise to get block temps back up and never another issue.

Pressure check the cooling system (pita but can be done). Run the T stat and properly vent the crank case would be my suggestion.