Aquanox and Aquanox 2 are available off steam as well, for cheap (though not that cheap).
Aquanox 1 is a good game, and I enjoy it greatly though the Schax are pissing me off no end. The music is also incredible. Aquanox 2, however, is more character-heavy and doesn't play as well for it, the music isn't nearly as much fun, though the combat feels better. (It's also not a sequel; it plays out at roughly the same time.)
However, there is a little-known fact: these games are sequels. There was an earlier, German game (it was translated as well, but I've forgotten the name; I assume it's incredibly abandonware by now) that was essentially the best aspects of Wing Commander and Privateer in the same setting as the Aquanox titles, using the main character from Aquanox 1.