The one piece of feedback I'd give is that I'd like enemy spawn rate to ebb and flow a bit. As it stands, the spawn rate only goes up, which leaves me feeling like I basically have to be manning the guns at all times after a bit. If there was a sine-wave-ish ramping up and down--let's say, with about 30s between the trough and the peak--that gives me time to go buy upgrades and/or repair the drill in relative peace...but then, will I get caught off-guard? (I'd recommend having the actual time between peaks vary each time, so the player's always guessing how much longer they have.)
Also: Is there a depth at which you reach another dock and can go back to the surface? Knowing this would affect my strategy as well.