If the story I heard is correct, we are all using train paint.

The 2 part polyurethanes(IMRON) were developed for to paint railroad cars because the harsh environment. Race cars picked it up because it could stand up to the sand blasting so you did not have to repaint as often. DuPont made some changes so it looked better (less like railroad paint) and a race car painter figured out how to give it that wet look. Custom car painters picked it up and it skyrocketed.

As far as I know there is no significant difference between marine, auto, and aircraft single color 2 part polyurethanes. The online help for DuPont Spies Hecker, or PPG is usually happy to tell you which version is suitable for boats and what primer to use for maximum durability.

I added the single color because I just discovered there is a 2 color 2 part polyurethane auto paint. From what I am told most factory cars use this system. It requires a clear top coat or a transluscent colored top coat. Since it rained the entire week of July 4th here, I refinished a bicycle using a sample kit I was given. It does not seem to be as durable as IMRON.

NOTE: When I finished this I started thinking, I bet the reason we hear about problems with the water based 2 part paints like System 3, is they do not have anywhere near the tech support that DuPont, Spies Heacker or PPG provides. When I called the tech support for DuPont they were able to tell me the best way to apply their paint over epoxy composites and they asked "is this hand layup or prepreg?"