I used to work for AL Boom diving and they were very professional.

Mmmm no comment!
In the Muscat area the best diving is at the Damaniyat Islands.
Extra Divers I believe are re-locating or maybe have already relocated to Barka from Al Sawadi Beach Resort, and should be around 15-20 mins away. They use small boats with outboards
Other option is Oman Sail and Dive at the Millennium Hotel just north of Al Sawadi, nice big boat with proper dive platform at the stern, check out other threads in this part of the forum.
For Mussandam, there are lots of choices, I have been invited on one this weekend, which I have to decline as I will be flying on Sunday. This trip is for two nights (Thu/Fri) with 6 dives on Fri/Sat full board for AED3,000.
There are also day trips, with several ops running fast boats to Lima Rock and that general area, however if you are leaving from the Omani side you have to go through a checkpoint that has in recent years got more complicated and requires some serious planning with invite letters and copies of passports at least 24-48 hrs beforehand.
Some days have been absolute shambles with waits as long as 2 hours to cross that border point, so be prepared. Several of the east coast ops run trips to Mussandam, better to choose one that leaves from the UAE side of Dibba if that is now at all possible. Contact Nomad (in Oman), and Divers Down, Blue Planet, Freestyle to name but a few, check this link for your research
Dive Centres | Emirates Diving Association
UAE east coast diving is much better than off Dubai, which is mainly wrecks with unpredictable visibility.
The east coast has three wrecks in recreational depths (Inchcape 1 - 30M, Inchcape 2 - 20M and Inchcape 10 - 24M). Other dive sites include, Dibba Rock - 14M, Martini Rock - 19M, Hole in the Wall - 14M, Coral Gardens - 22M, Anemone Gardens - 25M, Car Cemetery - 16M, Sharm Rocks - 14M. I would recommend Blue Planet or Freestyle for Dibba Rock (5 min boat ride) and for the rest Divers Down except for Inch 10 which is off Fujairah, better to use Coastal Technical Divers if they are doing that wreck as they frequently dive the Ines (70M) on a Friday but Saturdays are usually non-technical.
Have a good trip