For my upcoming trip to Thailand, are there any good scuba diving spots on Ko Samui? We'll be there in March of 2013.
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:
Lonely Planet tends to agree:
LP mentions Ang Thong too, but not in "Diving" section but "Other water activities":
However, slighty contradicting itself, the "Do" section on Wikitravel also says:
...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.
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.