Dive schools and charter companies are located mainly in Bophut and Chaweng. Although the dive spots tend to be an hour or more speedboat trip away, most of the dive companies are based on Samui rather than Koh Tao, as from a tourist's perspective, there is more to do on Samui so it makes a better base to explore the area.
Many dive companies offer PADI dive courses, with training in resort swimming pools, before venturing into the Gulf of Thailand’s clear waters. Once you are qualified, join a dive charter to explore excellent diving locations not far from Samui. These include Ko Tao, also known as turtle island, Sail Rock as well as the Angthong Marine Park. See tropical fish, rays, dolphins and corals and even the elusive whale shark is not an uncommon sight on dive excursions.
The Samui archipelago is generally good for diving most of they year, other than during the monsoon season (Mid October to Mid December) when visibility decreases, and currents are stronger. Dive by the motto, 'Take only photos, leave only bubbles' and respect the underwater environment. Take all rubbish away with you, and leave shells and coral where you found them. If every diver had to take even one souvenir, there would be nothing left for divers in the future.
Always practice safety when diving, and go with a reputable company. Bangkok Samui Hospital on the ring-road in Chaweng houses a hyperbaric chamber for the treatment of divers' decompression sickness.