Is a beautiful little island a meager 8 hours from bangkok. Take a train from hua lamphong to chumphon. Train is around 400 baht for second class sleeper fan (do not take second class air con) or 1200 first class. Buses are anywhere from 270 to 800 baht, did not have huge problems with the cheap bus from sai tai talling chan i.e. the southern bus terminal on putthamonton sai one. When you arrive the ferries run from 7 am to 1 pm for 600 baht and take an hour and a half. There is a midnight boat that costs less and takes 6 hours, but really recommend train that gets in about 5 am and then just buy the boat ticket at the station if you arrive at night take a motorcycle taxi to farang bar and either stay there or any of the hotels around it. Farang bar can arrange the boat the next day.
The boat will make a small stop at a private island before arriving at koh tao. From the pier take a left to get the guest houses. There is an amazing walking street out of mae haad with a ton of bars and discount diving shops.
Diving is really cheap 800 baht a trip with gear or 8000 and up for a 3 day diving registration course. Snorkeling is 680 baht for 4 stops including that private island you saw earlier (100 baht to get on the island)
Water is fairly clear and snorkeling is fun, but do not go during the rainy season run off from the hills will impare visibility. The best time to go is around may to october.
Rooms run from 400 to 2000 baht a night. The island has a lot of resorts. Over all very cheap accomodations and ridicilously cheap diving.
Koh tao is fairly clean for thialand no apparent red light districts or massage parlours (although the aloe vera massage is highly recomended).
The coral is starting to bleach, but the island has banned plastic bags the seas are fairly clean in other words. Really a nice and surprisingly very cheap experience. One the best holidays in thailand.