Diving / snorkelling in Brac, Croatia

I'm off to Brac, Croatia in September. We're staying in Sutivan. I've tried to find a web-site for a dive centre in Sutivan, but with no luck. Can anybody tell me if there is a dive centre in Sutivan, and is it likely to be open in September ? Also, what are the best diving and snorkelling spots on the island of Brac ?

Thanks.