Kingdom Hearts 1 - PS2 - Atlantica Part III

After the
next cutscene, head all the way back to the Undersea Valley again, save
if you will, grab the Dolphin again, and go back to the Sunken Ship.
Sunken Ship
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Shark. Behind one piece of the ship against the wall is something that should
catch your eye. Sebastian will push it for you, revealing a new cave.
Den of Tides
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There's a small room, the Cavern Nook, with another save point (inside the
larger clam), and another way leads to the Tidal Abyss, and...
Ursula's Lair
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Time for a boss battle. Ursula is tough...until you get the secret. Watch
the color of the potion she throws into the cauldron, and match it with the
appropriate magic. This will both damage and stun her, giving you ample
opportunity to smack her around.
If the cauldron is glowing blue, hit it with Blizzard Magic, if it's red,
go for Fire. If you hit it twice right before she's about to finish the
spell, the cauldron will glow white and she'll be knocked dizzy. Be careful
of her spinning spell, smack the eels to regain your health, and generally be
patient rather than aggressive in this one.
You'll get the Merman Kick as a new party ability, so be sure to activate
it (double-hit X/Circle or square and hold). From there, make your way back
to the main Calm Depths, and then use the kick to propel yourself towards the
light at the end of the cave with the flowing water. At the end of the cave
on the right is a tunnel labeled "???". Take it. Of course, you might want to
save first.
Open Ocean
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Ursula bad. Sora smash Ursula.
It's almost that straightforward. Dodge her attacks, get to the side (or
even the back), and smash away. Loads of fun. Having a Thundara Ring or two
equipped can save you from the constant lighting she throws at you, and Aero
can help withstand some of her attacks as well. Watch out for "That's for
being pretty", which precedes her huge lightning attack. Nasty.
After the battle, you'll end up in Triton's Castle with Ansem's Report 3 and
a Thunder upgrade to Thundara. From there, head back to Ariel's room to
complete this world with your key and get a new Sora weapon, the Crabclaw.
Finally, you can get out of those Goofy-looking (pun intended) getups.
On the other side of Agrabah is Halloween Town. Let's visit.