For my upcoming trip to Thailand, are there any good scuba diving spots on Ko Samui? We'll be there in March of 2013.
All the good dive sites are close to Ko Tao but if you want to stay on Ko Samui you can easily book a day trip from Samui and they'll take you to the Ko Tao sites (more expensive than from Ko Tao). Sail Rock is considered the best dive site in the Gulf of Thailand. But talk to the guys on site to give you more advice.
Haven't been there myself, but drawing from Wikitravel's Ko Samui guide and my copy of LP: Thailand's Islands & Beaches (published July 2010), here's some information:
The usual panoply of watersports are available, including plenty of dive shops, but most diving is done either in the nearby Ang Thong Marine National Park or Ko Tao as the visibility around Samui's sandy beaches tends to be poor. You can book diving day trips at dive shops, most of which are based in Chaweng. The dive boats tend to leave from the pier at Bophut and Bang Ruk.
Lonely Planet tends to agree:
If you're serious about diving, head to Ko Tao and base yourself there for the duration of your diving adventure. If you're short on time and don't want to leave Samui, there are plenty of operators who will take you to the same dive sites (at a greater fee, of course). Try to book with a company that has their own boat (or leases a boat)—it's slightly more expensive, but you'll be glad you did it. [...]
Certification courses tend to be twice as expensive on Ko Samui as they are on Ko Tao, largely due to extral petrol, since tiny Tao is significantly closer to the preferred diving locations.
LP mentions Ang Thong too, but not in "Diving" section but "Other water activities":
For those interested in snorkelling and kayaking, book a daytrip to the stunning Ang Thong Marine Park.
However, slighty contradicting itself, the "Do" section on Wikitravel also says:
The sea visibility in some places around Ko Samui is almost very good (distances of up to 10-30 meters). One can enjoy splendid sights of underwater mountains, coral gardens, undersea rock formations, hard and soft coral and whale sharks when in season.
...and goes on to recommend Samran Pinnacles (for experienced divers) and Ko Kra & Ko Losin.
So, if "almost very good" is good enough, check out the places around Ko Samui. :) Otherwise, if your schedule allows, head to Ko Tao for scuba diving.