It was said earlier in this thread, and to be honest I'm surprised it's just been suggested once, so I'll make it again: Subnautica. It's one of the most atmospheric games I've ever played. Sure, it all happens underwater, but that's the amazing thing about it, and the environment is utterly stunning. It's still early access but it is a true gem, completely fleshed out and working (has been for the last few years as well) and just keeps getting better. The way it looks now vs how it did a few months ago is an utter transformation.
There are worse ways that you can spend £20 to be honest, and it'll no doubt drop to a lower price in the upcoming Steam sale.
If you like underwater, keep peepers on Debris.
Just been delayed a bit, but looks promising.
Some clips on youtube play show bits of start.