Aquanox is one of the most underrated games out there - its awesome. I will say that not everyone will like it though.
I first heard about Aquanox in an article in PCgamer. I am a big Descent fan, so the first thought that popped into my head was (sweet, it's descent underwater). I didn't hear about the game till a couple years later when I found it for 7.99 with the obscure bargain games in KB Toys. I bought it, and I am glad that I did.
Aquanox seems like an action game, but it is really much more than that. Aquanox is a work of art - a fantastic underwater world that makes itsself believable while still managing to poke fun at itsself with the over-the-top personalities in the colorful locations.
If all you want is action, then Aquanox isn't for you. There is a lot of dialogue in the game that tells the story (a very good one imo), but there is a lot more of it that just adds to the atmosphere. You will either spend a long time between missions, or you will skip through it and wonder what's going on. The game is a package deal, you have to play it the way it was meant to be played in order to get the most out of it. I've heard some people complain that the game is too easy - if you think that please try the quick mission against the guy who stole the Succubus - the final boss is pretty tough as well. The subs don't move super fast, but they are subs after all, and some of the action can get really intense.
At it's best though, Aquanox is one of the most immersive games out there. The graphics were the most amazing in their day, and they still hold up pretty well today. The world just sucks me into it, and I know there must be others who feel the same way and can appreciate the masterpiece that this game is. I couldn't put it down a couple years ago, and i loaded it up the other day and I am hooked again.