Head towards the ship, which is to the top right of the shore and when you control Tidus talk with Wakka and the others. Talk with Wakka again and talk with Yuna for a while. You’ll soon find out that Braska’s guardian, Yuna’s father, was Jecht, Tidus’ father…but before you can talk any further the ship is attacked. Defeat the Sinscales first, but as another wave comes at you a dialogue appears prompting you to attack the fin.
This battle shouldn’t be too hard. You can use Kimhari’s Lancet attack to attack from a distance as well as Yuna’s summon. Her summon could die in this battle, but if you can have the summon use it’s Overdrive attack, you can deal up to 1100 HP of damage to Sin. Just use Lulu and Yuna’s summon to deal magic attacks to Sin. Take out two of the three Sinspawns since if you take out all three…more will attack you. This battle isn’t too hard. In the next battle after Tidus falls underwater and Wakka saves him…avoid the Sinspawns. They will come in an infinite number, concentrate your attacks on Sinspawn Echuilles but keep your HP above 200…otherwise the Blender and a few charging attacks from the Sinspawns will get you.
As the battle ends so does the shores of Kilika as there is little you can do about Sin’s attack but watch.