Warframe [official site] already looks like a game about sealife thank to its exotic power armour designs - with hammerhead helmets, lobster armour, unspeakable many-eyed horrors from the depths, and pearly nacre coating everything. Actual underwater combat was inevitable, I suppose, or all those watery warframes would get crabby. Crabby. Like a crab. That's a joke.
The latest expansion, Echoes of the Sentient, has added underwater action in new levels and, for land-lubbers, new parkour moves. It's all out in the free-to-play co-op shooty dungeon crawler now.
The new water tileset is introduced with new levels that are partially on foot, partially underwater. Leaping into the sea (at appropriate moments) will transform you into Archwing mode, which means your warframe wears a big bubble-blowing backpack as you zip around blasting everything with your warguns.
Back on land, warframes have picked up some swish new parkour moves. They can double jump, air dash, latch onto walls, dash up and along walls, and sprint along ziplines. It all looks very swish.
The expansion also added a new warframe, new weapons, new items, a new boss battle, and plenty more detailed here. You can gawp at some of the shiny new things (and some of the devs) in this video too: