When I first started diving, O2 kits weren't standard kit on all dive charter boats.
The club I was a member of had an O2 kit, which used to go on all trips, some skippers had O2 on board and some didn't. Now, you expect skippers to have O2 on board ,or you vote with your feet and book an alternate. We still take our kits regardless. If nothing else, some one might develop symptoms after they have left the boat, so having an O2 kit with the team is a good thing.
When I first started diving we had to be specially certified (the BSAC O2 Course). I think in the early days, it was viewed as diagnosing and giving a 'drug' (medicine) to an injured party. Which, a member of the public cannot do in the UK. Hence, 'certification' that you where appropriately 'qualified' (to give O2 to divers only). Instructors had to have additional certification, to teach O2 administration. Now, teaching people to use O2 equipment, is within the core diver training program, and any BSAC instructor is qualified to teach it.
I think I've 'used the kit in anger' twice. Once we had a full on bend. Shortly after the diver returned to the boat, he felt some pain in his arm, further examination, he had a skin rash. He went on to O2 immediately, and we started a treatment log, and re-tasked the boat back to harbour. He deteriorated steadily, then stabilised. He was helicoptered off Orkney back to the recompression facility in Aberdeen, where he spent an evening in the chamber. (Full recovery).
I think the second occasion someone was feeling uncomfortable after getting home after diving, so he when't onto O2 then did a walk in to the London hyperbaric unit after a conversation on the one with their medics.
As a club, our kits have been used multiple times, normally for other divers on sites, near drownings, bends, abnormal ascents etc.
I think over time, like O2 kits, it will be expected that the boats have AED's on board.
If nothing else, it's extremely reassuring for the rescuers to have an AED. One of the great advantages, is they check for heart rate, they will only 'work' if they can't detect a heart rate.