Hope I'm not too late to reply. In Koh Samui, most people stay in the eastern beach named: Chaweng as it's the longest and most beautiful one. North beach "Bophut" and "Maenam" are less popular but cheaper, and what is good about it is you can see the beautiful sunset there. As for diving trip, there are a lot of qualified shops doing the business on the island. You won't miss them if you take a walk a look by the beach.