Where do you go after you go to ariel`s grotto in atlataca?

There are a couple of times that you go to Ariel's Grotto in this world, but I'll assume that you mean after the first time.
As soon as you leave the Grotto, you'll get a cutscene involving Flotsam, Jetsum, and Ursala. When it's done, you'll be in the Undersea Gorge. You need to fight off a bunch of heartless, then find the fish and swim around with it for a bit. When you're ready, go to the Undersea Cave.
Fight off the heartless here, then go through to the Calm Depths. Again, you have some heartless to defeat. After you beat them, swim into the current going upwards, and you'll be in the Undersea Valley.
Here, go around the area and defeat all of the heartless. After that, grab onto the fish that you grabbed onto in the Undersea Gorge. The fish will take you to the Sunken Ship area.
Swim into the ship to start a cutscene. When it is over, go towards the treasure chest, and wait for the shark to eat through the bars. When it does, grab the treasure in the chest, then leave the ship. Now you have to battle the shark.
I'll leave it to you to figure out how to beat the shark, and what to do after that.
If you have anymore questions, just ask here, and I'll try my best to answer them.