Boss Battle: Sin's Fin and Sinscales
Sin's Fin is main target here. It has 2000 HP, and each Sinscale has 200. You need to knock off the 2000 HP the Fin has to progress the battle, but you won't want to do that straightaway. Each Sinscale you kill will net you AP, so you can theoretically get a huge amount of AP and Sphere Levels from continually killing the Sinscales. Each Sinscale will take two physical attacks from Tidus, Wakka or Kimahri, or one of Lulu's black magic spells. One critical hit from Tidus, Wakka or Kimahri will also do the job nicely. Kill off about 30-50 Sinscales (you won't regret this) then get Lulu and Wakka in the main party and finish off the Fin. If any of the Sinscales' wings start to flicker, make sure you destroy them first since that attack can hurt. Before you finish off the Fin, make sure Wakka and Tidus are healed as well, since you will need to be for the next stage of the fight. Use Yuna's Cure to do this — try to reserve Potions Don't worry about the Fin either — it's "attacks" will be to move from side to side and replenish the Sinscales once the three on deck are killed. One final thing: do not use Wakka's Element Reels if he gets an Overdrive. Save it for the next battle.
Once you've dealt the damage, Sin's Fin will pull the harpoon gun clean off the ship, and Tidus must have went overboard. Wakka will jump in after him, and help him out. But unfortunately, it's not over yet. A monster appears — time for stage 2!
Boss Battle: Sinspawn Echuilles and Sinscales
Not using Potions in the last battle was important because here it's just Tidus and Wakka with no other methods of healing. If you have Wakka's Overdrive, use it now (element doesn't matter, although Thunder seems to work well) and make sure you get three of the same to hit every enemy. Otherwise, do not bother attacking the Sinscales. They keep respawning. Instead, focus on Sinspawn Echuilles. Use Dark Attack to blind it, which will make Drain Touch miss often. Each of the Sinscales will do a sort of torpedo attack which does about 60 damage, and Echuilles' Drain Touch does 60 to one person and heals the damage dealt from himself. Blender, his only other attack, does about 110 damage to both Tidus and Wakka. Between using Dark Attack, just physically attack Sinspawn Echuilles and use Overdrives when you get them, but heal when either of your two characters goes into critical health (yellow). It won't take too long to beat Sinspawn Echuilles. You can do the same with the previous battle and repeatedly kill Sinscales for AP, but this isn't advised since it's a lot easier to die in this battle than the previous one.
After this battle is finished you'll get the rewards — if you did kill a lot of Sinscales, somewhere in the region of 60 Power Spheres, about 1000 Gil, between 2 and 6 Sphere Levels for everybody, and if you're lucky, some assorted equipment too. The next scene is a sad one, after which you will be off the boat, and in Kilika.
