Sub^3 aims to be a realistic multiplayer submarine simulator. The goal of Sub^3 isn't to be the best sub sim out there, but to have the best sonar modelling of any civilian simulation available. To accomplish this, it will use USML for all acoustic simulation. Sub^3 currently uses SFML for graphics, networking and input. It also uses SFGUI for the user interface and Boost to support the multiplayer networking code.
Sub^3 is playable, depending on how you define "playable". You can do very basic submarine hide-and-seek, but that's about it. Development takes place at https://github.com/nopoe/Sub3