Aquatic Revolution

From the introduction:

This adventure can be can be easily customized to a party of any size or level, or run as a level 6 adventure for 5 players. It can work as a small side adventure in an existing campaign, as a standalone one-shot, or the start of a new adventure. Use the links to the tools in their relevant section to customize the adventure to your party.

The PCs are traveling on a boat across the ocean. The players will need some form of water breathing to do this adventure, so either a spell caster with access to 3rd level Water Breathing, a scroll or potions that they find, or you can hand-wave it and make the dungeon breathable.