You can definitely get a solid dive computer for under $400, and the $1000+ are going to have features that won't be present on the Apple Watch Ultra, like air integration, lower depth ratings, etc.
Paying other people to take you diving can get pretty expensive quickly, but the dive professionals themselves generally do not make a lot of money. It is one of those things that people do professionally because they love it, not because it pays well. (Speaking only about recreational diving here, industrial diving is a whole different beast.)
Also, a quick glance, dive computers seem to be $400+, many are $1000+. The watch in question is $800 and I feel like people who are into diving aren't the type of people who find an $800 prohibitively expensive.