You have to ride on a dolphin/use dolphin kick to get past the currents
Back behind a piece of sunken ship where the large sunken ship is, there is something that Sora can't reach. Sebastian will offer to swim back and push it. Then the door to Ursula's lair will open.
With your gummi ship.
You fly there in the Gummi ship.
When air is pumped into a sunken ship, it acts as a floater. The air boosts the sunken ship's buoyancy and helps the trapped water to escape.
The time line is: Kingdom Hearts: Birth by Sleep, Kingdom Hearts, , Kingdom Hearts: 358/2 Days, Kingdom Hearts: (RE) Chain of Memories(at the same time), Kingdom Hearts II, Kingdom Hearts III: The Keyblade Wars.
Kingdom Hearts is a fairly long game. Kingdom Hearts has many things to be accomplished such as Gummi Ship routes, the various worlds found in the game and the optional bosses.
It is still the same as in Kingdom Hearts. The left analog stick steers, X shoots, O rolls.
The Gummi Ship feature was added to Kingdom Hearts II with extra abilities, weapons and an easier cutomization mode. Traveling to different worlds was also made much easier.
First after you speak to arial in her grotto and king trident destroys it you should see a cutscene of ursala talking to arial u then go to trident and see he was overcome. Sebastian then follows u and go back to the sunken ship. examine the big rock and Sebastian will remove it. go in. Ursala after that in in a cave across the sunken ship cave now since u have dolpin kick. note:this answer might not be completely true
"The sunken ship sat on the bottom of the ocean."
shipwreck, sunken ship
the ocean