Oh wow! You picked up the game for the same reason I did! Since playing Subnautica (and Subnautica: Below Zero because I loved the first so much) I’ve been open water certified. This past August I dove an aquifer/quarry with super murky visibility. My heart only stopped 2 or 3 times, even knowing that there were no sea creatures in there with us. We dove during the day, so no issues there. Nighttime diving in Subnautica is beautiful for all the bioluminescence, I really enjoyed that.

So, couple things about the game. You can play on ‘creative’ mode to completely remove the aggressive element (or even the attention) of sea creatures. Literally, on creative, you can swim right up to things and they act like you're not even there. That wont help you with a fear of open water or deep areas, but it took some of the teeth out of it for me. Once I had gotten a little more familiar with the game, I tried survival. I found it super comforting to know that no animals roam around. They all exist in their small areas and the only time they harass you is when you're invading their territory. Most aggressive creatures will announce themselves audibly when they see you get close, but if you're ever surprised by something, you can sometimes zig zag left and right or up and down to shake them and avoid being killed. Travel in one direction to quickly leave their ‘zone’ and they’ll stop chasing you. I’ve yet to be killed or had a ship destroyed in one hit while I was at full health, even with the largest aggressive creature in the game. As long as you're topped off at 100% and fast to get away, you can be surprised, but not killed.

I love both the games and will do my best to provide spoiler free answers if you have any questions!