Yeah I agree, it looks really cool.
Doesn't look too dark to me but it's always hard to tell with screenshots. Could be your gamma, my gamma, etc...
But everyone is right, make sure it's got enough light to see players easy.
I think you should finish it, I'd love to see some glass tube walkways between bases. Much like the first shots but maybe more open with a pitch black ocean out there. Maybe someone could animate some fish to swim by the glass.
And you've obviously put alot into it and the details. Finish
You could even have water dripping in at tunnel seams
I'd suggest NOT having a door on the intel room though. That could get ugly running through a door into a heavy or sentry and no way to tell until it happens.