Haad Yao Divers
Haad Yao Divers runs a dive shop right on Haad Yao β the long west-coast beach that's also one of the island's best sunset spots β which makes it the natural pick if you're staying out that way and don't want to travel across the island just to book a dive.
The boats themselves actually leave from Chaloklum pier on the north coast, so there's a shuttle transfer built into the trip β a small extra step, but one that gets you to the better dive sites on that side of the island (Sail Rock included) without needing your own transport sorted first.
It runs the standard course range alongside fun dives for already-certified divers, and its location makes it a convenient option if Haad Yao, Haad Salad or Mae Haad is your base for the trip rather than Chaloklum itself.