In the RMSS (and I suppose RMFRP too) rule book, it is stated that a maneuver roll is required only for actions that require concentration or are done "under pressure".
Since see-dwelling creatures are used to swim like we are used to walk, I'd say that you juste have to roll for swimming when in a stressfull situation or when the PC is trying to push the milits of its abilities.
Just like when you roll a distance running, sprinting, climbing, etc... MM with a walking creature. You won't roll for walking 10 meters, you will for running at top speed for 100 meters or if you are chased by something hungry and run for your life.
This said, I don't understand why a sea dwelling creatures should use a talent to breathe under water Oo. If the race description says or implies it does, well, it does. You need talents to do things that are unusual, not for usual things.