Subnautica. I didn't really get it at first. Swam around the life pod a bit; didn't see the point.
Was travelling some time (months?) Later and stuck at an airport with my laptop and a Power outlet but super patchy wifi. So I fired it up again and really, really got into it. Kept playing on the plane and all through that trip. By the time I got back, I had a Cyclops and a Prawn suit and was about to find the lost river.