I'll try to give you some honest and accurate answers.
Generally, the diving in Musandam is very good. The fact that Dibba is less than 2 hours from Dubai makes it a great choice for a weekend or day trip. Places like the Damaniyat Islands or Salalah whilst offering great diving, are a lot further (especially Salalah which is a very long way).
The marine life in Musandam is diverse, hard corals, soft corals, small stuff, bigger stuff, sun-fish, whale sharks, black tips, leopard sharks, hammer heads (further north). Generally the fish life is well established despite the best efforts of the local fishermen to keep the fish market at Dibba well stocked. Sadly, the fish market is the one place that you are pretty much gauranteed to see sharks.
Water temperature at this time of year is around 23-24c. It will go up in the middle of summer (July/August) to around 29c (air temperature around 50c with 90% humidity). Visability can be hit and miss. I have dived in Musandam and had 20m+ visability, but I have also had 2m vis.
As for Sheesa Beach, I have only dived with them once, but would have no problem rebooking some more dives with them. Service was good, and my only criticism would be that the speedboat that tags along with the dhow was too small for the number of divers on the trip. This was one of the comments I made on their feedback form.
As well as Sheesa Beach, there is also Al Marsa. another company that runs trips into Musandam. You could also contact some of the dive centres and see what trips they have booked etc. Divers Down is a very nice dive centre on the east coast, located not far from Dibba. Contact a member on here called Aquamarin. She is from Divers Down.
Hope that gives you a bit of the info you were after.
