CocoTipAdventures

Phone-first operators like this one are how a lot of Koh Tao still works. There's no website, no booking portal, just a number to call and someone on the other end who'll talk through what you want. If that sounds like friction, it is. It also tends to mean smaller groups and a guide who adjusts to the day's conditions rather than running the same loop regardless.

The draw here is the water. Koh Tao's reefs and dive sites are why people get on the boat from Chumphon or Surat Thani in the first place, and an island-based outfit is positioned to make use of them. Go with a clear idea of what you're after, whether that's snorkeling, a coastal run or something longer, and ask directly. Vague inquiries get vague answers.

December through April is when the sea is calmest and visibility best, which is also when the island fills up. Book earlier than feels necessary in those months.