Tunnel-diving RPG UnderRail has its own waterworld as of today, as creators Stygian Software released an expansion named Expedition. Taking place partway through the base game, it opens up a giant underwater lake called the Black Sea (no, not that one), over and around which players can scoot on a range of jetskis, and fight sea drakes, pirates, and dormant robots, and meet a society of boneyard dwellers.
This "secondary campaign" adds over 250 areas, three new weapon classes, and a heap of new character feats, items, and craftable gubbinses. A trailer entrails below.
After much initial optimism I got bogged down in annoyances and struggled to enjoy UnderRail, but its efforts to recreate but also modernise the much-vaunted original Fallouts were not in vain, and the underground sea setting could add an interesting new flavour and a curiously Ultima-ish tint to its leather-clad proceedings.
An update to the base game also addresses one of my biggest complaints by (optionally) speeding up the game outside the turn-based combat sections, alongside the usual tweaks and pokes. You can read more details on the game's site.