If you’re planning on using the new Apple Watch Ultra as soon as possible as a dive computer immediately after you get it, well, just hold off for a bit.
The actual Oceanic+ app that turns the watch into a dive computer is not yet available and won’t be available until later this fall, as Apple said in its initial announcement (even if that detail was buried deep in the press release).

The watch does feature a Depth app that’s available at purchase designed with an intuitive user interface. With just a glance, the Depth app displays time, current depth, water temperature, duration underwater, and max depth reached. Users can program the “Action” button to instantly launch the Depth app.
That Depth app, though, should absolutely not be used as a dive computer, according to Apple:
“Underwater activities are risky. The Depth app isn’t a dive computer and doesn’t provide decompression stop information, gas analysis, or other recreational scuba diving functionality. If you’re using Apple Watch Ultra where the failure of the device could lead to death, personal injury, or severe environmental damage, always use a secondary depth gauge and timer/watch. Before going on a dive, be sure your Apple Watch Ultra is free of cracks or other damage.”
Check out how to (safely) used the Depth app here.