After leaving deep sea research you can go back for a difficult dungeon. Ultima Weapon is at the bottom and there is a GF to draw from him. The enemies along the way are pretty difficult as well so it's not for the faint of heart and the save point near the bottom is invisible so you'll want to use Siren's Move/Find ability to find it and save before the boss.
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~C~ FD
https://imgur.com/dGDfxaw