Haven't read the book enough to know, but I recommend against a one-hour rule for several reasons.
Firstly, it will annoy the player. One hour? That's a lot of game time. People like being inconvenienced by their character's quirks, but not being cut out entirely for hours at a time.
Secondly, it doesn't make any sense. The logic seems to be that, to maintain good health, sea folk need to stay hydrated and keep their gills wet. Once you're soaked, you're soaked. It wouldn't take an hour to get gills wet.