The Underwater City is the only underwater island in Blox Fruits. It's located between the Frozen Village and the Prison.

How do you level up fast in Blox Fruits? We’ve sailed the seas ... He’ll give you the instructions to defeat the Ice Admiral at Frozen Village. Complete the task and return to the Military ...

Blox Fruits First Sea Map In Blox Fruits, you can either become a Marine or a Pirate. Depending on that choice, you either spawn in the Pirate Starter island or the Marine Starter island.