Soph
i am right now in Koh Tao
a number of things to point out
1 alot of angry trigger fish here
2 the vis is real bad at the moment but that's due to the trade winds i think, so the water is stirred up quite a bit.
3 stay away from Ban's or budda view, and go with a smaller operator, where you will get better attention to your questions etc.
you have just missed the whale shark season
but they will be back at some stage.
dive sites
cheapon pinnacle
twins
south west pinnacle
shark island
sail rock
red rock
mango bay and ah leuk are nice bays for open water trainning
these are good sites but depending on vis again, twins has a lot of triggers around so stay sharp, but then theirs sebastion to see, and his clever trick
caraboe as mentioned earlier is a nice relaxed DC but so is dive point, which has the best boat on the island, and u don't have to climb over 3-4 boats to get to yours, as like with budda and asia divers.
anyway all the best when you get here