Koh Tachai
25 km north of Koh Bon , the divesite is a bit out from the island so strong current can occure. Koh Tachai dive site is not the easiest in Thailand but even here, mantas and shark are spotted.

Distance: 14 NM north of Koh Bon
Estimated Travel Time: 90 min
Depth: 6-50 m
Visibility: Good to excellent
Current: Moderate to strong
Surface condition: Can be wavy
Level of experience: Advanced and higher

Liveaboard with at least 3days/2 nights
This site is little to far for a daytrip(it is doable!!)
But most trips include this with their Similans and Koh Boh trips.