One of the Oren City Pirates are holding a secret message in a bottle. An illegible treasure map I am sure Isaias in Rionnag Harbor can help decipher. If you were to find this, read it and take it to Isaias. He will guide you to find those hidden treasures in the sea and maybe even a way to explore the rumored Atlantis!

When he sends you to the pirate ship, you might get swept into the brig if you seem too suspicious. Take some important items from the pirates in the brig such as a bandana, brig key, and ship keys. It will help you get out of the brig, stay out of the brig, and unlock a door in order to find the two items Isaias requests (make sure you have space in your inventory for 2). Of course, Isaias will need some items to fill this tall "order" according to the schematics! Once you bring him the letter and schematics, bring him the following:
 1 Flawless Ruby

10 Human skin

2 Breath Sack

25 Rum 

What is the rum for? I don't know. But don't make him angry, just bring him the rum. The Oren Jungle blacksmith in Jungle 15 should have some for sale. 

Once you gather those items, he should have the scuba gear ready for you. Get to exploring lynith sea for a prized treasure chest or set out to find and explore Atlantis, the home of the merfolk.