Apple Support, or rather, Apple Legal:
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.
Snark aside, this does seem like a really cool feature. I’m no diver, but I’m looking forward to getting my Apple Watch Ultra tomorrow.