That will have underwater aspects as well?

I was thinking about this, swimming by itself is nothing to write home about but things are a bit different when not only can you dive underwater but there's actual underwater combat as well. It looks like you won't even be limited when it comes to using magic underwater either.

Now to me, it seems like if they put this much time and effort into something like this, they plan on using it for a more than one world.

I doubt Atlantica will be returning.  I can see other worlds coming into play.

Yes and no.

Yes I think there will be another "water heavy" world. I agree it's weird that they would put in the time and effort to make underwater fighting animations for just one world (The Caribbean), which could hint at world like Atlantis the Lost Empire or Finding Nemo. This is completely different from the flying fighting animations in KH1, that were just essentially air combos that never let you fall to the ground.

No I don't think there will be a "water heavy world". While most of the confirmed worlds aren't water heavy, they still have bodies of water that Sora could fight in. San Fransokyo is situated in a bay, Kingdom of Corona has a large roaring river, and so on. Sora can even break some ice in Arendelle and swim under frozen lakes (though this would probably kill him). Remember also, Atlantica in KH1 had battle animations exclusive to that world.

You don't necessarily need another water focused world to take advantage of the underwater fighting mechanics.

You make good points. Though i would think if they were taking advantage of underwater combat for other worlds, we would've seen some more footage of that.

Like with all the footage we've seen of Corona, don't think we've seen anything on that aspect. I can see BH6 not bothering with it since the only body of water is near the bridge area and i can see it being unreachable because of the loading/zones.

With KH1, I would think it wouldn't be nearly as expensive and time consuming to do then compared to now. Though the only reason i even brought this up was the off chance that with the implementation of underwater combat that it could raise the chances of