Dead Man's Chest (5E)

From publisher blurb:

No place offers more adventure than the ocean. No dungeon is as deep, no jungle as full of exotic and dangerous life. Most folks spend their entire lives on dry ground, unaware that entire civilizations thrive beneath the waves, sometimes far more ancient and steeped in mystery than any on the world’s uppers surface. Though player characters have long grown familiar with the air-breathing world above, seldom do they venture into the depths, and when they do, they discover wonders they never dreamed existed. The ocean offers a venue for adventure that is at once alien and appealing. Strange things exist down there, as perilous as they are compelling.