Enemy list
Chest list
You have the Crystal Rod. Time to sail for the Mysidian Tower and claim the Ultima Tome. Something interrupts the trip. 5 chests inside, all permanently missable.
From Cave of Mysidia, sail north toward the Mysidian Tower. A cutscene triggers when you get close.
Inside Leviathan
1F. The dungeon is short and completely linear. The floor deals 1 HP per step; not dangerous but it adds up. Bolt Fish and Manta Rays are the main encounters. Manta Rays hit hard in groups, so keep HP topped off. Grab Chest 1 (Twist Headband) and Chest 2 (Power Sash) along the path before heading to the next floor.
2F. You'll find a Dragoon named Ricard further in. Show him the Crystal Rod and he joins. Head north of him for Chest 3 (Power Armlet), then continue to the top left exit.
3F. Chest 4 (Gaia Blade) sits at the top of the map. Chest 5 (Diamond Shield) is on the south side; Red Souls guard it but they're easy. None of the enemies here are missable. Head to the middle left for the boss.
Roundworm
Beat the worm and board the ship. Ricard stays as your fourth member; he's a solid physical attacker with decent starting gear.
Next stop, Mysidian Tower. The Crystal Rod unlocks the door, and the Ultima Tome waits at the top.