Any HN scuba divers here that would use this as their only dive computer on a dive? I'm not a scuba diver myself but do engage in other adventure sports where reliable equipment can make the difference between a great trip and disaster. I'd be very reluctant to trust my life or health to this.
Absolutely not! Apple's hardware keeps getting better, but their software reliability has been quite bad over the last 10-ish years. A dive computer is a safety-critical machine - an apple watch is not suitable as a primary unit. Maybe if you're doing only very casual dives and you don't mind the risk of having to do an emergency surface, it is fine.
You call the dive and surface slowly. Maybe you don’t do a second dive if you can’t be sure of your limits. It’s an inconvenience.