Boss - Poseidon
So this guy is pretty tough, no lies there. But let's face it, you've been preparing for him since you got your ship. I sort of took a gamble with this fight, but it worked out quite well. The first thing you need to do is use the Trident, since Poseidon is protected by some weird forcfield. Once that's taken care of, you can start battling him just like any other boss. Just be mindful that Poseidon is VERY offensive since he can kill you in one turn and rarely does anything to help himself now that his forcefield is gone. You're going to need either Healing Aura, Wish Well, or a Spirit Ring to help you through this battle. Piers was a pretty good choice since Poseidon is strong against water and I wanted Jenna to be attacking with fire instead of healing. Sheba would also work great with a Spirit Ring since she's fast and you could then use Ulysses more often.
Chances are someone will die. Make sure you distribute your Waters of Life to your team evenly since Poseidon can kill you at all angles. Watery Grave can be an auto-death, Ocean Fist can rob you of all but 1 HP, and Counter-rush can finish off any low HP teammates. It also helps a ton to raise Resistance, Defense, Attack, and Agility. Along with healing, that's all you need to pound Poseidon right back into the depths of the sea!