The term “Dive computer” is not something to be taken lightly. It’s hardware you literally depend on to keep you alive during a dive. The fact that Apple is supporting this externally developed app says they have a very higher level of trust in their hardware.
Typically a dive computer would cost more than an Apple Watch and be a single purpose device. The fact that Apple is entering the arena is quite a disruption as other brands will be forced to either compete on price or really raise the bar on innovation to justify their current levels.