Just returned to Seattle after two weeks of diving in Musandam from Khasab. From my perspective the best sites are closer to Khasab than Dibba: I think Extra Divers in Khasab is the only regular operator to hit sites on Musandam Island, Kahyl Island, Abu Sir, Pipi Beach, and Beecham's Bay. The Dibba operators we encountered were further south in areas like Habalayan Island. It is really worth the extra hour drive to Khasab to dive the northern sites.
We spent one of the weeks doing reef surveys with Biosphere and Emirates Diving Association. There were no signs of red tide in any of the areas we dove. While most of the coral was in great shape, there were signs of damage from the typhoon that hit a couple of years back and the subsequent extended red tide event.
What a great area to dive and visit: excellent fish, invertebrates and coral. Really outstanding geology. We found the Omanies to be most gracious hosts and guides.
Dan