Subnautica - Unknown Worlds Entertainment's excellent aquatic survival adventure - is currently free to grab and keep forever on the Epic store, making it probably the most tempting reason to look at the shop after exclusives like Hades and Ashen. Released less than a year ago after a lengthy ride through early access, Subnautica is a gem. As someone who doesn't ordinarily like survival craft 'em ups, I still got into it, exploring its thoughtfully designed cavern networks and alien ruins as I delved deeper underwater. The game is free for the next two weeks, so swoop on down and grab it for free before it goes back up to £19.49/€20.99/$24.99.
As Brendan said in his review and Fraser did in his big list of great survival games, Subnautica is a more peaceful game than most in the survival genre. Alien worlds are hard enough to live on, and it's even worse for humans when there's almost no land. There's some plot reasons for it, but you're never given more than the absolute bare minimum to defend yourself, and usually non-lethally. Giant predatory beasts under the waves can be driven off briefly with concussion torpedoes, but you're not going to be cutting a bloody swathe through the ocean, so best learn to live alongside it.
The first couple hours of Subnautica are probably its weakest, although feeling your way around its hand-crafted (as opposed to procedurally generated) environment is exciting. You'll initially flounder around in the shallows figuring out what you can and can't interact with, what each tool really does and how to extend the range of your all-too-small initial oxygen tank. You've got a little escape pod to fall back to in the early game, but once you're acclimated to the waters you'll probably want to build yourself a home away from home beneath the seas. Maybe decorate it, or adopt a pet - the heartmelting 'cuddlefish' will be your loyal underwater buddy, if you let it. You should.
It takes a pretty beefy PC to make Subnautica truly sing, and the game has some rough patches, but overall it's one of the better survival games out there, and costs absolutely nothing - you've got no excuse for not picking it up. All of this is a good way to warm up and get your feet wet before the standalone expansion Below Zero arrives. It looks pretty cool, pun obviously intended.