Yes. Yes Yes Yes. YES YES YES.
I think this would make the game one of the first games to have some kind of naval/underwater warfare aspect. I mean, as cool as aircraft and tanks would be in PS2, they would be just as cool as any of their counterparts in other FPS games. Speedboats/submarines/warships/aircraft carriers/"scuba diving" /amphibious tanks/water bases, would all be pretty much original delivering a different experience, unlike any other FPS game out there.
Now I am not saying that it should be released at game launch, though it would be cool to have it as an expansion (probably with the introduction of new "naval" continents with water areas, other then the boarders, with water bases and stuff OR just saying that now the factions have discovered that water is not solid and that they can dive in it, so they started building new bio-labs and resource collecting bases, basically just expanding the water area (at the borders) and putting some underwater bases in there.) even an year later or something to bring a new and exciting thing to the game, an experience that cannot be experienced in any other FPS.