Explore the ship and talk to everyone. Below the deck, you can find a merchant, O'aka. Not Wakka. He overcharges a lot, but is nearly everywhere in the game. If your in a good mood, give him some gil. Just give him 1 gil and he'll be happy. Also, buy some stuff from him. Then go in the power room and you'll find the Al Bhed Primer vol. III. Then go talk to Yuna.
It won't take long after talking to Yuna that everything gets deadly. All of a sudden, the ship starts to shake, and people run onto the deck. Get ready for a fight too. After the crew harpoons Sin, your taken into battle.
Sins Fin
HP: 2000, AP: 10,
Steal: Nothing
3 Sin-scales are on deck. Kill off 2 of the sin-scales. If you kill all of them, Sin just gives you more to kill. With only 1 left, bring in Wakka, Lulu, and Yuna into battle. Have Yuna summon her brand new Aeon (Valefor). Every action an Aeon does will increase its overdrive. So attack Sin with thunder. When your Overdrive fills up, unleash the Energy Ray onto Sin, most certainly killing him. If the Aeon cannot pull it off, and dies, then start attacking Sin with Wakka�s normal attack and Lulu�s magic. Heal with a potion the second you drop below 200 HP. Sin won't take long to beat.
Sin is scared off, and not long after, Wakka notices Tidus is gone and a Harpoon is missing. Wakka dives into the water only to find...
Sinspawn Echuilles
HP: 2000, AP: 12,
Steal: Nothing
Now you only have Wakka and Tidus, so first things first. If you drop to 200 HP, heal up. And always make sure the enemy has Darkness inflicted on it. So try to use Wakka�s Dark Attack every 3-4 turns. Keep using your normal attacks, and your overdrive whenever needed. Heal when needed and keep up the fight. Do NOT attack the Piranha, just the Sin Spawn. After plenty of attacks, it goes down.
