My dad is physics teacher and... He said car mechanics are... awful, he is also developer and said that creating real physics isn't hard, so I think Rockstars developers are just sh*t.
My dad is a meta-physics teacher and he said the car mechanics charge too much. He is also, coincidentally, a developer and a rocket scientist, airline pilot and leader of an L.A. gang and said he knows your dad and that your dad got it wrong... again, but that you should totally cancel your pre-order because because none of us want you in the online game.
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.)
Regardless of the engine, a 3-D model and its textures can easilly be reused and repurposed. As for the quality of those models, I don't see any difference in quality between what I've seen in the previews and what was in L.A. Noire (which I found to be very detailed.) And, while L.A. is very different today than it was in the '40s, it was built on top of what was there before so landmarks like Griffith Observatory, the Hollywood Bowl, and Union Station all still exist and will almost certainly find their way into GTA V. And this is to say nothing of the bungalows, storefronts and thousands of tiny items that were in the game.
Of course, that doesn't mean they haven't built these things from scratch, but why re-invent assets when they could spend that time building new ones?