It's probably got something to do with the game developers being lazy and not coding a part past the first with Wakka and Tidus.

After you can actually return to the area, if you jumped in, it probably would not change your party, which means the animators would have had to animate underwater encounters for ALL members, which of course, is not possible storywise, nor is there any Save points in this area for at least 4 screens.

Also, the Pirahnas cannot be captured, and if you had a capture weapon, it would catach them. There's no treasure in the water that cannot be bought from stores afterwards.

Also, in the cutscene, Tidus and Wakka don't actually leave the water. They just walk up to the pathway, which suggests this scene was a one time event only, similar to the Salvage Ship (you can't get back on it), and most of Bevelle. You don't actually see your character leave the area, you only see them arrive in the next.