I thought Gothic 3 had, hands down, the most ear pillaging voice acting of any game out there ... but Aquanox is giving it a run for its money. If it weren't for how hilarious El Topo sounds, with his bouncing between Sean Connery and some kind of Boston accent mid sentence, I think I'd turn the sound off during the conversations.
The voice actors (and I use that term very loosely) were obviously given one take to read lines they'd never seen before and that's it. And so far I'm pretty sure all the female characters have been voiced by men trying to sound like girls (it isn't working).

It doesn't really help that the protag is the "straight man" and everyone else is some kind of ethnic stereotype.

Also, the writing has a pretty bad case of "If All Stories Were Written Like Science Fiction Stories". ( http://www.shrovetuesdayobserved.com/flight.html seems to be unable to handle the traffic at all, sorry.)

There was a particular female character early on that thankfully only appeared in one conversation. Nearly made my ears bleed.

Then there was that character that injects/smokes anything he can. If I ever meet his voice actor in person, I cannot be held responsible for my actions.

I installed Aquanox 2, and maybe it's because, after Aquanox, anything else sounds great, but the VA in that game is SO much better. Its not AAA quality or anything, but I think Massive actually hired voice actors and let them read the script a few times before they put them in the sound booth.
I've only played maybe 3 missions from each game so far, but A2 seems like the better one for sure.

Aquanox is actually somewhat "moddable".

I remember back in the day, I used a tool called unmpak to unpack the sound files (they are stored as mp3), and used a replay-gain tool to reduce all voice to volume 0. (Replacing them with blank mp3s won't work, since the dialog relies on the timing of the voice files.)

Essentially I muted all the dialogs in the game. Since it is fully subtitled, this makes the game MUCH more playable.

(I may post a mini-tutorial on how to do this in a later date.)

While I agree that the voice acting in part 2 was much better, it's main character is such an annoying little twerp whose lack of anything resembling a spine makes me want to reach into my monitor and throttle him. He's the kind of guy who deserves every bad thing that happens to him. Better voice acting can't make up for an unsympathetic character.

Apparently personal weapons were never invented in this world because, if someone had taken my vessel and was stupid enough to allow me back on board I'd turn the whole thing into one big Die Hard Sequel. just sayin'

did you somehow change it in the end or did you play the game in english?

because i too remember the incredible german voice acting and find the original voices to be less good. i mean, they're not bad, but far away from what the german VA did.