Enemy list
Chest list
Walk north from the overworld into Vikings' Cove. The Nepto Dragon has been terrorizing the bay and the Vikings can't sail. They'll give you the Enterprise ship if you deal with it, but there are 6 chests and a shop to hit before you head out.
Vikings' Cove
No hidden items in the cove, but the walls are full of secret passages. From the entrance, follow the path until you spot a chest on the right. A hidden passage at the top middle of this area leads right to 300 Gil (60/367).
Head north and take the stairs up to 2F. On the left side, there's a secret passage to the left of the cannonballs. Walk through it, go south, then down the stairs to find 2 chests with 300 Gil each (61-62/367). A healing spring sits in the corner of this room.
Back on 2F, head right from the stairs. Between the cannonballs on the right side is another hidden path leading to 3 chests: Thundara (63/367), Fira (64/367), and Blizzara (65/367). These are Level 3 spells, so you'll only get 1-2 casts each at your current level. Spread them across 3 different mages instead of loading all three onto one; that way each mage gets a cast during the Giant Rat boss fight in Nepto Temple.
Before leaving, visit the shop on 1F and grab a few Maiden's Kiss items. They're your only cure for Toad status and you'll need them in later dungeons. Rest at the healing spring, then exit from the northeast of 2F.
Nepto Dragon
Talk to the Viking leader to get the Enterprise. Board the ship and sail north until the Nepto Dragon attacks. At 60,000 HP it's the same kind of unwinnable fight as Bahamut at Dragon's Peak. Run on your first turn and the bestiary entry registers automatically.
After fleeing, a cutscene reveals the dragon's eye has been stolen from its statue. Sail back to shore and walk north toward the temple entrance.
Nepto Temple
Cast Mini on your party and enter through the dragon statue's empty eye socket. Like Tozus Tunnel, physical attacks deal 1 damage while shrunk, so switch to an all-mage party if you haven't already. Put everyone in the back row to cut incoming damage. And remember the Fire Staff you picked up in Tozus casts Fire for free as a battle item (no MP cost), which helps save your -ara spell charges for the boss.
Nepto Temple has 3 chests and 5 new bestiary entries:
Interact with the dragon head to drop down to B1. Follow the path through to B2, where you'll see two holes in the floor. Take the left hole first for Shell Helm (66/367) and Shell Armor (67/367). Climb back up and take the right hole for the Serpent Sword (68/367), then drop through the next hole on the right to reach B3.
Petits can petrify your characters, so don't ignore them. Poison Bats inflict poison and Blood Worms drain HP; kill both quickly with Fira or Blizzara before they wear you down. Poison Bats are the rarest of the five enemies here and mostly appear on B2, so keep fighting on that floor until one shows up. If your MP runs dry after collecting all 5 bestiary entries, leave the temple to rest and come back for the boss. Follow B3 south to reach the Giant Rat.
How to Beat Giant Rat
The Giant Rat drops the Nepto Eye. Backtrack out of the temple, remove Mini, and place the eye in the dragon statue to receive the Fang of Water. The seas are safe and the Enterprise is yours to keep. Switch your party back to your preferred jobs and save.