Depends if your idea of "good for night diving" consists of a scene from Star Wars! It's not uncommon to have a half dozen boats tied up there.
The standout sites of Koh Tao IME were Chumphon, South West and Sail Rock. If you're lucky there may still be a few bull sharks hanging around. There's a bit of macro to find as well- sea horses for example can be found... if you know in which direction to swim. The constant crashing around by student groups means they are normally not to be found on the other training sites.
Normally you can get a decent deal on accom by saying either at the dive center backpacker, or at one of their 'friends', especially if you sign up for a AOW or Resc course.
To appreciate the outer reefs like Chumphon and SW, you need to leave 45mins before the other dive boats. If not you're jumping in to bubbles. New Way Diving opposite the 7-11 at Sairee used to leave early in order to do this- not sure if they still do now.