PADI divemaster (ret'd) just chiming in. Man, this 'Ultra as dive computer' could be a can of worms for Apple without serious study & understanding of dive physics. Not only that, an understanding of all the everyday diving 'use cases' that people get into (good or bad) as they gain experience.
In other words, experienced divers know full well that yes, time, depth and tank pressure are all, of course, essential. But that's just three elements in a very complex tapestry of data that divers have to parse and understand to maintain full situational awareness.
'Recreational' divers come in all shapes, sizes and experience levels. Many will routinely 'push the limits'. Safety stops are advisable at 10 feet for all divers. Multi-leveling your way back up from depth could save your life, or kill you if you run out of air.
My favorite dive computer was brick called
The Edge. It drew a little pixel bar graph of your dive profile, with time on the x-axis and depth on the y-axis. A parabolic line showed the limit of your theoretical tissue loading. No air integration, that's what your pressure gauge was for. Time remaining at one's current time and depth was constantly updating as time and depth changed. Fabulous for deeper dives, scooters and multi leveling. That was the data I needed back in the 1980s
Until the Ultra h/w and its available s/w can provide that degree of sophistication I wouldn't advise anyone to even call it a dive computer. It's a watch with a depth gauge. Diving is, and always will be, fun. But it's got to be treated as "serious fun". Jacking another feature into a watch without a deep understanding of the issues doesn't strike me as wise.