There are hundreds of fishing boats at various sizes down at Cai Rong Pier. It shouldn't take long to find a fisherman that is willing to help you out. Some of the fisherman may collude to offer you an obsurd price. It shouldn't cost more than US$30 to hire the boat and it's owner for 6 hours. Remember fuel is a huge cost for the owner so the cost is naturally going to be quite high. Bai Tu Long Bay is generally very calm because the many islands protect it. So even the smallest boat should be safe enough.
Check out the floating fishing villages. Get your personal captain to show you some caves. Climb the hill on the island with the big dragon. Go for a swim. Watch the sunset over beautiful protruding limestone islands.