I was diving in Mussandam about two weeks ago ( around March 5). We travelled by Dhow out of Dibba and travelled up the coast for about 2 1/2 hrs. Red tide was most notable at Dibba marina and continued north for about 1 1/2 hours (sorry, don't know the distance). There were also patches of red tide around our dive site.
I typically dive in / around Carribean, so diving here was a bit different. I asked several times about expected visability, DM and clients all said it will be fantastic/beautiful. The best you can find anywhere. I found the vis to be about 20 ft...then I realized the other divers were from Great Britain, Canada, etc. and told tales of learning how to dive with 5 ft vis...so it's all relative. 20 ft vis to me means I'm cancelling the day's dive and I'm hanging out with a rum punch, listening to the local band...to other folks it's fantastic and they travel long distances to see 20 ft. So fair warning.
I started the dive trip, scheduling with an operation called Diver's Down (Khorfakkan, Fujeirah). Nice folks. The UAE Coast Guard suspended all diving (due to the weather...hmmm not a cloud in the sky, flat calm...another UAE moment...apparently this has been happening frequently)...Diver's Down arranged for us to travel up the coast to Dibba, Oman (bring passport), Dhow Boat ride up the Omani coast and diving. Nice day. Diving not spectacular, water is cold (3 - 5 m would be nice)...nice variety of fish...nothing spectacular, not great volume, ....really cool bioluminescent jelly fish (in the daylight, but shadow of cliffs)....which I spent too much time with and ended up with souvenier JF rashbut. oops. Interesting school of squid hung out with me while I did my safety stop.
Good luck on your explorations. Another note, I attempted to go diving with the Abu Dhabi dive operation, but when I arrived at the dive boat, they didn't have a ladder for exiting the water...no kidding....I'm a disabled diver, so I had to cancel my trip. I've heard vis off Abu Dhabi and Dubai is 5 ft due to the construction of the islands...reefs destroyed etc. I heard this from local divers who have stopped diving anywhere in UAE due to lack of visability.
Hope this helps, have a great time.