If you were at Rezzed last week, you had two opportunities to see Failbetter folks live on stage, talking about narrative games on a panel hosted by our own John Walker, and presenting a talk on the Zubmariner expansion to the wonderful Sunless Sea [official site]. You can watch the latter presentation below and it contains loads of new details about venturing into the depths of Fallen London's perilous waters. Key points: the Constant Companion is the most horrible of all the horrible things, and submarines are fragile things.
Zubmarines then! You'll be able to get hold of your very own submersible shell fairly quickly once you've purchased the expansion. It's not given to you automatically but it won't take a great deal of effort to get hold of it. More interesting still, the zubmarine form is an extension of your ship, so if you've got a beast of a machine already, it'll morph into a beast of a zubmarine.
The lure of the depths is similar to the lure of the sea itself - hazardous, lonely and terrifying though it may be, there are treasures to find. And more importantly there are stories to discover. Curiosity is the fuel that drives those ships.
Other than that, it's best to let Failbetter speak for themselves. They have quite a way with words. As does that chap at the beginning and end of the video, doing the introductions and hosting the Q&A. That would be me.