High season on Koh Tao usually starts in June and runs through July and August. So if you are planning to learn to Scuba dive or do some fun dives, it is best to pre book to avoid dissapointment. This should also be said of your accommodation as at this time of year, hostels, guesthouses and hotels can be fully booked. Dont forget to book your travel ahead of time as well, if your comming from Bangkok, Koh Samui, Chumphon, Surat Thani or Koh Phangyan book your travel as soon as possible. Koh Tao does get full at this time of year but the same can be said for many places around Thailand, be aware that it is the monsoon season now on the west coast of Thailand, so many travelers and holiday makers are planning their holidays already for the Gulf of Thailand, where the weather is usually much better. Book early and come dive which Scuba J