This review contains no plot spoilers, but mentions mechanics and items you may not want to know about if you want to go in completely blind.
Subnautica was one of my all time favourites. Thankfully with it's huge success, the team have taken everything they learned and produced an even more special game with a larger budget. Everything from Subnautica is improved. If you enjoyed the first one at all, this is an absolute must-buy. I played Subnautica 4 times (which is ridiculously rare for me), most recently 2 weeks before Below Zero came out to remind myself. This review is, as always, a simple list of good and bad, with the addition of what I noticed changing from the first game.