Here is a datapack designed to create a world of oceans and islands for exploration.

The Overworld is a dimension of tropical seas dotted by jungled volcanic islands and paradisaic atolls. The seas are littered by wrecks and the beaches rife with buried treasure left behind by the gold-loving pirates who ply the shallow coastal waters.

The Nether is a dimension of deep seas shrouded in mist and permanent twilight, from which islands rise steeply, skirted by craggy basalt cliffs.

The End is an alien dimension over the edge of the known world, where exotic materials are found in abundance and strange creatures guard the alien oceans and fungal fields and jungles.

Upon spawning into the world you will find yourself on a small island. Volcanic islands can be found using the locatebiome command and searching for island:volcano. To find an atoll island use the locatebiome command and search for either island:exposed_reef or island:lagoon, or use 'locate' and search for a village. This is advised as the islands are a long distance apart by boat. If you don't spot a pirate ship early on, use locate:igloo to find one.

TO INSTALL:
In the 'Create New World' menu select 'Data Packs'
Click 'Open Pack Folder' at the bottom and drag the the-treasure-islands.zip file into the folder.
Back in Minecraft click the datapack in the 'Available' list.
Choose 'Done' and create your world.