the mermaid kick where it made him swim faster
You get the Mermaid Kick when you fight Ursula the first time. If you are having trouble finding her, follow these steps.
"Now head back to the Sunken Ship area. Enter through the hole to the right of the entrance to Ariel's Grotto.
Head back to the rock with the weird symbol on it. (Which is by/in the back the ship, I believe that your little crab friend points it out to you.) When you check the thing behind the ship next to the rock again, Sebastian will help you. Enter the Den of Tides area. Stay to the right as you pass the fork to enter a room with a Save Point and a MEGA-ETHER. Make sure to equip any abilities you may want, and any armor you feel is necessary. For Sora, put Fira in his shortcut menu. Make sure you're around level 34 as well. Finally, make sure everyone has plenty of Ethers, Potions, Mega-Potions, and Mega-Ethers.
When you're ready, head on out and take the other path in the fork to reach Ursula."
If you have already got the Mermaid Kick and fought her the first, just equip it to Sora and then ride the very fast current and head to the right, you should be standing right in front of a hole. Equip any Lightning Resistent to Sora and then go and fight her.
