I'm an arachnologist and I agree that it looks a lot like Desis martensi, although I'd need to see the spider to be certain. Staying submerged for so long doesn't fit their known ecology, but maybe the spider was retreating to the top of your tank - do you see any silk up in the corners or the overflow where the animal would have been out of the water?
Also, just curious...where did you get your live rock?