http://store.steampowered.com/app/465670/
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STEAM KEY REGION FREE GLOBAL

The year is 2110 and humanity depends on secret marine farms for survival. Created by a philanthropic organisation through a century’s tireless work, the aquatic farms are under threat from predators emboldened by an ecosystem in chaos. Cybernetic fish-bots were created to keep this precious food source safe. Then it happened: a glitch? A hack? All you know is that the fish-bots turned from guardians to enemies. Now you stand between the human race and extinction.

In this retro-styled side-scrolling shooter, you pilot your sleek submarine through levels by using keyboard (CTRL to shoot and SPACE to deploy one of your special weapons) or gamepad. The future is a difficult place to find resources and you must earn credits and pick up materials as you conquer levels, then use them to upgrade your ship to match your playing style, or take control of a different model. Select one of three kinds of special weapons to take with you into each battle: depth charges, torpedoes, or nukes. Learn how the world came to this crisis as you collect log entries from those who lived through the decline of the planet, and by compiling them unlock the location of Earth’s final hope, the main research lab of Giga Corp.

[Update] as of today 17th June Multiplayer is now a feature. Using dual control pads you can share a screen and battle your way through all the levels.