Hey David beautiful project by the way- I’ve used Hempel – Bilge And Locker paint with massive success. When I first moved on to my Jaguar 27 the paint under the half berths was flaking off and covered in thick black mold. I had the luxury of being able to to get under there with a jet wash on a hot day and blast it off. The place got soaked but it did the job quickly. I then cleaned with a large sponge. Lastly I did a light hand sand over the entire area that I was going to paint.
I used an electric fan heater to dry the area and applied Bilge and locker paint which is a 1 part enamel. After 2 years of sleeping in their /kicking getting damp it held up amazingly. you shouldn’t need a primer and it’s fairly thick and will only require 1 thick coat with a second for patching up.
I personally would only ever use that now having had amazing results. I’ve used white but would advise a light grey.