Keep in mind that scuba diving comes with the limits of an oxygen supply and the deeper you go, the less time you have with your air. Diving also gets exponentially complex as you go deeper, with the danger of nitrogen narcosis increasing rapidly at depths beyond 100 ft.
As for the submersible, these two have limits, but even if the game one doesn't, it could easily be handled the way boats or plans are, with the same artificial limits. (I'm still surprised this wasn't handled by land on the east side of the map with an endless desert.)
That's real life. GTA V is not real life. I highly doubt we'll have nitrogen levels to deal with. Same with Oxygen levels. IGN or CVG would have mentioned it, but they have never mentioned it to my knowledge.
Anyway what I'm wondering is, will we have a flashlight or something we can use when scuba diving? We know we can put them on guns, and the sub itself has lights, so I'm hoping we'll have one to explore those dark areas, like ships, and buildings from what I've heard, fully without it being dark.