Koh Tao certifies an estimated 80,000+ divers per year — the highest density anywhere in the world.
The geography is ideal: protected coves on the west side with 5–10m sandy bottoms perfect for Open Water training. Reefs are within 15-minute boat rides. Multiple dive sites cycle through different days so courses don't get stale.
The economy revolves around diving — shops cluster in Sairee Beach with shared boats reducing costs. Accommodation, food, and bars all cater to dive students. Easy to meet other divers and continue training.
Reef condition is decent but not Thailand's best (Similan Islands win that). Visibility 10–20m typical, occasional whale shark sightings April–October.
For non-divers, Koh Tao is small (21 km²) and quiet — fewer party options than Koh Phangan but easy day trips. The trade-off for the dive-centric vibe is limited cultural attractions.