Sunless Sea's upcoming Zubmariner expansion is looking wonderfully eerie, like pretty much every game set under the sea. Sorry, the 'Zee', which is like the sea but zingier, probably. Developer Failbetter Games offered a brief glimpse of life, and lights, under da Zee back in January, and here's another trailer detailing the Zee floor.
Regular Sunless Sea is set on top of the water, while Zubmariner is set beneath it, which means reduced visibility, weird freaky fish, and the odd light to help guide your way (or lead you astray). You'll pilot a Zubmarine, which comes equipped with a handy sonar function, augmenting your view.
A big new thing in Zubmariner is interactible objects, which you can...interact with as you explore the Zeebed, in exchange for a bit of story or maybe some loot. There's no date for the DLC yet, but if you've not yet taken the plunge on Sunless Sea, it's in the current Humble Bundle for not much money.