As I understand t he OP, he is planning to paint
over powder coat, and thus etching the alloy has already been done!
I'm no expert on powder coatings, and there may be many different plastics used in them. I'd contact the dinghy's manufacturer and ask what they are using, and for advice on painting. I suspect that sanding
off any gloss and then a coat of Vinygaurd or other vinyl primer, followed by antifouling will work
All the advice about how to coat bare alloy is interesting, but not applicable here.