Failbetter Games just released a two minute video showcasing some of the new gameplay that will be coming to Sunless Sea with the game's Zubmariner expansion. In Zubmariner, instead of sailing on the Zee you're diving beneath it, exploring the floor and bottoms of islands.
You can watch the video here:
The expansion has no release date yet. Editor Daniel Tack reviewed Sunless Sea and gave it an 8.0, saying "Sunless Sea is an interesting game that’s worth playing for those that appreciate a dark yarn, players that love exploration, and even the 'let’s find out how to game the system perfectly' roguelike crowd. It’s a unique mix of genres that’s a finely crafted experience, marred by a few cracks in the cabin."