Albeit at that sort of depth is it not completely, utterly pitch black anyways? With powerful lamps only lighting a few metres?
Yeah, but on the other hand, computer games shouldn't enter a serious relationship with reality anyways.
But it turns out it's already been done, by a German company, and it looks so gorgeous that we decided to leave it to the pros. Try it if you can find a copy, it's not bad.
I own a copy of that game, and it sucks, hardcore.
But where there's water, there's life, no?
Not necessarily. Water itself isn't the source of life, you could very well end up with an ocean that doesn't contain the necessary organic compounds to develop life on some planet.
You could build a decent submarine fighter with todays technology, but it's neither needed nor feasible in current conditions.
And lasers work underwater too, though some wavelengths are especially affectected by the water, and the absorption is pretty high so they'd be quite short ranged in could be essentially useless in certain conditions.
Looking at the speeds, FS craft's speeds are about right for fast submerged vehicles, IIRC the highest conventional speeds you can achive in water are about 80-120 km/h.
One important thing the engine lacks would be localized light sources - There'd surely be underwater installations with bright spotlights and so on, and some craft would mount lights too, and it'd be nice to have some sort of spotlight on your craft, too.
It'd be cool to have that one even for space based missions, the starfurys in B5 mounted some spotlamps too and it looked damn cool when they searched that freighter with them in the first episode. You could probably modify the code used for weapons illumination, or lightmaps like used for the players flashlights in older FPS could work too if you wanted to implement something like that.