http://www.pcgamer.com/stardew-valley-multiplayer-beta-coming-early-2018-underwater-area-teased/

In November, Stardew Valley creator Eric "ConcernedApe" Barone gave fans a glimpse at some upcoming singleplayer content, not to mention the game's long-awaited multiplayer update. A new teaser arrived today, but this time it's publisher Chucklefish that's poking the bear. See for yourself:  

The previous teaser image showed a boat with, Andy reports, some lights, boxes and barrels. The room in the latest image is a little more revealing. Its metal structure and aquatic backdrop could hint at underwater farms or other undersea content, which fits with the boat. The machine tucked away to the left, which looks to have some sort of gauge measuring either air or water pressure, also supports the idea.