Abzu should be played in one sitting. It's only a couple of hours long, but you will quickly find yourself immersed in the beautiful graphics and music.
The graphics are stunningly gorgeous. A lot of games have open worlds where you can watch wildlife, but I must admit getting to see ocean life is so much better. You literally swim next to sharks, giant whales, and tiny fish alike, all while swimming towards you next destination or just sitting and meditating, watching the fish go about their lives.
The score makes this game even better. The music plays with you, so it ramps up during big moments and calms down when you are in a safer area. I'm not sure it's something I would like outside of the game, but it fits perfectly with that's going on in-game and really adds to the feeling of immersion in the game.
There is no real story, so I've just made my own interpretation that this game is about a civilization that was destroyed due to environmental destruction and you are trying to restore your world and make it habitable again with vegetation and wildlife, but that's probably completely wrong. The game does do a great job of communicating what you're supposed to do without actually telling you anything. It's a really neat way of showing not telling the player what they need to do to move to the next section, and other games could really take a page out of this book.
While it's not a walking simulator, the puzzles are fairly basic and never take much brain power to overcome. There is a part where you have to do some minor "platforming" by avoiding bombs, but even if you blow up it really doesn't matter because you can't die.
Overall, the game was a lot of fun for a couple of hours. I don't think I would replay it, as there isn't much else to do than what you've already done, and I played this game on PS Now but I don't know that I would pay more than $5-10 for it due to the length and lack of replayability, but I did enjoy it quite a bit while it lasted.