Stardew Valley will go through multiplayer testing on Steam near the end of this year, and the feature should be fully implemented and released across all platforms in early 2018. You should be able to go fishing, while your friends tend to the farm.
Stardew Valley doesn't really need multiplayer, but a lot of players are interested in the feature, and yours truly would enjoy some co-op fishing, farming and dungeon crawling as well. The teams at Concerned Ape and Chucklefish are currently hard at work in order to allow for players to share their daily farming routines in early 2018.
"Shortly after you begin the game, Robin will offer to build up to 3 cabins on your farm. Each cabin will house a farmhand, controlled by one of your friends. Farmhands can do almost anything the main player can do. They can farm, mine, fight, fish, forage, marry NPCs and take part in festivals. Each player has their own inventory. When a farmhand is not connected, their inventory can be managed through a chest in their cabin", was explained in a blog post by the developers. Sharing is caring.
It looks like one of the players, most likely the host, will be performing a "protagonist" kind of role, with anyone joining in on a multiplayer session being able to perform any actions the main player can as well, except for deciding the direction the main plot will take, and things like deciding on when to rest or start festivals.
Inter-player marriage is also a feature the developers are working on. While local or split-screen multiplayer aren't planned for the near future, anyone who owns two copies of Stardew Valley and an internet connection will technically be able to marry themselves.
Multiplayer features will be going through beta testing on Steam later this year, and an update implementing the changes into the game is planned for sometime in early 2018.