Ariel kicked all the Heartless' butts and the Nobodies couldn't invade because it's under the freaking water, like unless Xemnas is willing to degrade himself enough to transform into a sea cow it's not happening.
But seriously I don't get the KH1 Atlantica hate, it's one of my favorite worlds. It's highly interactive, it feels like you're constantly finding out about new places and shortcuts to get from one area to another, and Ariel was great as a party member. The fact that a lot of your abilities are totally useless because you're under water is a) like obvious because if you've ever gone swimming you realize pretty quickly you can't do things the same as on land and b) sets the world apart from all the others, challenging you to rethink your usual tactics at a late stage in the game. It really pulled the rug out from under the player; if you rely on dodge roll, too bad, oh you liked jumping, not possible under de water.
I just wish I could replace Under the Sea with Isn't It Lovely?, then it would be perfect.