I know the region quite well and have spent a lot of time working on marine surveys off Abu Dhabi, Qatar etc. You can find some coral communities and seagrass beds in the vicinity of Abu Dhabi, but again I wouldn't necessarily go there specifically for the diving. Coral is sparse and scattered when you find it and much has been hammered by bleaching and/or dredging and development. In the best areas you will get a fair cover of Acropora, Favids, Platygyra etc, and there can be a reasonable amount of fish life where you get rocks. Water temperatures range from very warm in the summer 30C + down to below 20C in the winter. You have to get a fair way offshore to get any reasonable depths and there are a few wrecks around that can be dived. Viz ranges from 3m or so when it's stormy up to 15m +.
Better diving can be had around the Musandam peninsula- a few hours drive up the coast from Dubai. It's part of Oman, but border crossing and visas are quite trouble-free these days. Musandam has much more interesting underwater features- fjord-like inlets, strong currents in places, good corals, fish, dolphins and even the odd whale shark.
In the main part of Oman, you should try the Daymaniyat islands, offshore from Sawadi and about 3 or 4 hrs drive from Abu Dhabi or Dubai. Great coral and lots of fish, but again visibility is a bit hit or miss and can drop to a few metres if there is an algal bloom. All nice diving and some unique features, but can't rival the Red Sea.
Hope that's some help. Incidentally, looks like you're from Cape Town- I'm going to be out there for work mid-late Oct and hoping to squeeze a couple of dive trips in. Any recommendations? (already considering a cage dive at Dyer Island and the cow sharks in False Bay).
Cheers
Fergus