I only ever made one foray into the world of I/C when, as a boy, I scraped together enough money to buy an ED Marine Seagull diesel for my Aerokits Fast Patrol Launch which performed miserably on a Kako motor and dry batteries.
It was a temperamental motor and I never got it to work properly even on the bench where it would either lock up or just run for a few seconds before expiring. The compression setting was obviously very critical, as was the carb.
The nearest I ever got to a 'sea trial' was to dip the rear of the boat into the kitchen sink filled with water. I eventually got it started and it ran for several seconds during which time the entire contents of the sink were copiously distributed around the kitchen. My Mother, like Queen Victoria, was not amused.
I have stuck to electrics ever since.
Colin