AquaNox

If it weren't for the sun-shafts, (unreachable) surface above and auto-levelling, it may as well be taking place in space. There's little in the way of scene-setting for an underwater environment: There's a complete lack of wildlife until the storyline demands it and the clouds of plankton may as well be clouds of inert astral gas.
The missions are strict and linear; you have your set play area and no scope to venture beyond it. And the missions consist of the typical "blow this up" or "protect this" objectives. They also don't have checkpoints so if you fail then all your work is undone and it's back to the start of the mission.
Gameplay-wise it's a lot like the classic Descent, but with auto-levelling orientation as there is no separate roll control; however at no point are you required to find your own way as there are always waypoints telling you where to go.
The story is clichéd; you'll have experienced similar in other games, not just of the era.

The visuals are decent if you're a fan of the late 90's/early 00's look in games. I quite like the style of the era but modern gamers will likely find them underwhelming.
Your wingmen are both unhelpful and irritating as when they have their own roles in missions, they take their sweet time in doing it, with you having to stop and wait for them to get on with it. And when they are joining in the fight their dialogue options are limited and you'll hear the same soundbites over and over.
Speaking of which, the action music is atrocious. It's the most 90's soundtrack I've ever heard and it's completely out of place in this game.

Finally you'll also encounter numerous bugs: Vessels under your protection driving into things and damaging themselves, wingmen flying in circles when they're supposed to be doing something, your vessel getting stuck on nothing and unable to move, CTD when restarting a mission; and so on.
As a fan of the era and of aquatic games, I was looking forward to playing Aquanox. But it's just not good enough; not least because there's little to no sensation of being underwater and in the end it plays like a weak FPS.

This AquaNox is the original it's polished, old but gold.

It's a bit of a pain to set up, but the setting is very worth it. The voice acting is so terrible that it winds up being great, the music is pretty unique, and the art direction really makes this game stand out.

one of the best games you've never played.

pros:
- unique gritty underwater cyberpunk setting
- 90s industrial techno soundtrack
- silly technobabble
- cheesy story
- over the top voice acting
- tight fps controls
- works on my win 11 machine in widescreen straight out of the box
- heavy action focus, not a submarine sim

cons:
- little variety in ships and equipment, everything is a straight upgrade
- most missions boil down to kill everything that moves
- no checkpoints in missions
- the menu has no mouse support
- the story is quite cliché (mega corps, drugs, ancient evil)
- heavy action focus, not a submarine sim if that's what you're looking for