Last time I checked, almost all underwater maps in video games suck, do you think Fontaine will suffer the same fate?

If they put enough effort into it, then it will be great. Subnautica is a game that is almost only in the water, and that game is fantastic, it's just about making sure the movement feels fluid and controlled.

I think it will end up being something used a bit around but nothing too important. Just like the boat after Inazuma. Some zone here and there, a couple of minigame and all.

It doesn't have to be great, as long as it provides a decent change of pace.

But i hope it will be popular : i'd be glad if they expand underwater exploration to areas like Cider Lake, or between the archipels of Inazuma.

I worry it'll trigger nausea for me.

As long as swimming isn't slow and they don't force underwater combat it's fine.

It mostly sucks because underwater movement is slow and uninteresting. Jumping and sprinting is a big part of making movement fun, so it'll be interesting to see if they can make underwater like that. That's why flying is also so boring, you don't do anything besides hold forward and lose all forward momentum when you drop which feels super clunky.