Near the entrance there's a chest containing Iridescent Wind. At the first junction turn right. Defeat the medusas (they have few HP but use powerful magic so watch out and try to back-attack) and take Water Earring from a chest. In the next chest there's a Prismatic Crystal. Head left and face three groups of Hermit Crab Poos, then go further due north. A little bit ahead you will find an enemy called Cutlass Fish. It has about 200HP.
HINT: You can cure Paralyzed status by using an item called Mobility Balm.
At the next junction go right again. At the end you'll see two chests. One contains Generalist Heart, and the second one is blocked by a magical barrier. Go back to the main corridor and go further north. The next junction take the left path. Defeat a Broadsword Fish (use Shadow magic or try to Monster-Battle it with Hermit Crab Poo). At the end of this dead end you'll see a transformation box. It lets you transform one minor item into a better one (nothing special, really). Go back to the main alley and go right this time.
By the next junction you'll see a chest. Remember its location and go further due north, until you see a cliff. When you defeat the monster there, a chest will appear. It contains a Fish-shaped key. Take it and use it to open the chest you remembered. There's a Hexagonal Crystal inside. Get back to the cliff and use it on a shell to the right. You can now cross the water.
A few steps later you'll go through a checkpoint, which means that a boss fight is coming. Go right to find a Flara spell.
Boss - Hydtrattler
This battle is very easy, as long as you can cast the Ground spell (preferably with 2 characters in each round). It enables you to simultaneously attack all 4 heads of this beast. Make the remaining two characters heal the party and you'll be fine.
After the battle heal all the characters from Poison status and get the goods from chests: Amethyst, Cure-All and a Water Necklace. Go further to finally leave the cave.