If you look at the vast majority of model submarines, most of which free flood, you will find that the consturction often GRP, and the inside is usually plain resin and matt or cloth exposed to the water surface, and they have to deal with water pressure too. I've yet to see any adverse effects from this. Some used to apply a float coat to the inside, but it adds a lot of displacement, and is now generally considered to be unnecessary.
Full size boats tend to spend a lot more time in the water, so I would class them a little differently to a model which is likely to get dipped for a few hours each week at the most. I should add that applying gelcoat to hull is a bad idea unless you add a wax additive, because it is air inhibited and will remain tacky, and it's too thick to use for laminating with cloth or tissue anyway. If I use gelcoat, I brush it in on it's own, then apply tissue or cloth over the coating, once set using laminating resin.