In short
Koh Tao issues tens of thousands of diving certifications a year, a volume that puts it among the leading centres in the world. The sheltered bay, the shallow depth and the competition between schools explain the price.
The island is twenty-one square kilometres. Sairee Beach holds the schools and the nightlife; the coves to the north and east are quiet.
What it really costs
An entry-level certification costs less here than almost anywhere else, often with accommodation included during the course. That is the reason to come and the main budget item.
Outside diving, the island is cheap: housing, food stalls and scooters all cost less than on neighbouring islands.
What nobody tells you
Scooter accidents are the leading cause of injury among travellers here. The inland roads are steep and sandy, and many hire places refuse to cover damage. Check your insurance before, not after.
Choosing the school matters more than the price: students per instructor, the state of the equipment and the teaching language make the difference.
The November–December monsoon closes ferry links for days at a time and sharply reduces underwater visibility.