I was diving in Snooky in April and described it to freinds when I returned as pretty similar to Pattaya: Terrible viz, no fish. Actually, Pattaya has an order of magnitude more marine life. Some pretty coral though.
I knew I was in for a boring dive when the guide said it would be a sand dive. Not even a reef. Yawn. Not my style.
Good luck with a shop there. There's three already and, in my opinion, the area cannot support 4. I think the only way to take over the market and really do well would be to come in there with an operation based on a fast speedboat that gets you to the sites in 30-45 minutes instead of 2-2.5 hours.
Sihanoukville is a great beach resort but who on holiday wants to get up at 6:30 to be at the dive shop by 7/7:15? Not me. That was way too early. Get a fast boat and you can start trips at 9, which is better for the customer base which is 99.9% tourists on vacation.
Sihanoukville diving has a long way to go, but a lot of potential if someone does something differnet and better than the three existing shops do.