These things change from region to region- you have the Middle East, where up till recently, Scuba was used down to 50m for buoying pipelines, etc, West Africa, which can be a half mask in the Warri Delta for the local chief out to Big time Sat and ROV's further out, etc. etc.
I can't answer for the GoM, which is where I'm guessing you are interested in, but as for the North Sea...
1. There is NO SCUBA, and no Recreational involvement in commercial operations Offshore (to be honest, the only area where you would get away with scuba in the UK is Aquaculture, clam Diving or media/police search). You would go a long way before you would find anything other than Kirby-Morgan on a Commercial job of any kind (the odd Genesis, maybe a Beat Engel).
2. For working on a Platform Jacket,
-either an Installed LARS system on the lower decks of the Platform, with 2 Baskets (one for Divers, one for Standby), with clump weights able to recover the basket on its own, or
-vessel based. This could be shallow surface orientated Air from a 'Daughter Craft' (usually a 40ft-ish RIB, with a Dive control built into the wheel house, and onboard gas storage- google KD marine and see good examples of these. They usually have a ladder from the vessel, and a winch for secondary means of recovery) or from a DSV on DP alongside the Rig. Daughter craft's usually require a DP vessel as a mother craft, unless it can be installed safely onboard the job, ie. FPSO or Semi-sub. Sat is always performed from a DP vessel.
3. Sat Vs. air is down to a variety of factors, Depth being a key factor, along with the work intensity, as when you get past a relatively shallow depth then the restricted bottom time, increased personnel and equipment required for 24hr operations means that most often, a Sat vessel becomes more effective, even at less than 20m or so. For a quick series of less- critical, shallow jobs, Sat can be much more expensive and unnecessary. Sat is favoured in the North sea as Surface O2 is forbidden, no planned decompression dives are allowed and Air over 30m is frowned upon. Air is used more often in the southern sector of the North sea, as there is huge tidal flows and relatively shallow water, meaning that most Bell runs might only achieve 1or 2 hours a tide, so you might only burn out two or three Air Divers a tide doing the same job.
(and money, obviously, but with oil at $110 a barrel, nobody is hurting too bad paying for a DSV to put Sat Divers down to get more out of the bottom)