“Rated to 100m” does not mean what you think it means. It means, essentially, that it theoretically would be safe at 100m depth, assuming you lower it slowly and don’t move it about at all. IIUC, local water pressure varies significantly when you move something around underwater, which is why “100m pressure proof” does not mean “can be brought down to 100m depth”.

    IIUC, local water pressure varies significantly when you move something around underwater

If Wikipedia can be trusted here, movement doesn't increase water pressure as much as we think: https://en.wikipedia.org/wiki/Diving_watch#Water_resistance

    Movement induced dynamic pressure increase is sometimes the subject 
    of urban myths and marketing arguments for diver's watches with 
    high water resistance ratings. When a diver makes a fast swimming 
    movement of 10m/s (32.8ft/s) (the best competitive swimmers and 
    finswimmers can not swim nearly that fast) physics dictates that
    the diver generates a dynamic pressure of 0.5 bars (50kPa) or the 
    equivalent of 5 meters of additional water depth. 

One other thing stands out to me...

    It means, essentially, that it theoretically would be safe at 100m depth

It's interesting (alarming?) that they chose the "weaker" ISO standard 22810 for water resistance, in which a sampling of units are tested:

https://en.wikipedia.org/wiki/Water_Resistant_mark

ISO 6425 is much more stringent and specifies that every single unit is pressure tested to 125% of rated capability, ie a 200M ISO 6425 watch has been individually tested at 250M for a period of time.

ISO 6425 is generally used for dive watch, eg a Rolex Submariner, not dive computer. I don't know any dive masters who wear a dive watch while diving; they seem like mostly a fashion / status symbol kind of thing? I have no interest in them anyways.

I looked at the Garmin spec sheets before buying my Apple Watch and didn't find any of their computer/watch combos listing ISO 6425. Instead they list a depth (usually 100M) and attest "Tested to engineering standard EN13319 for dive compliance", see https://www.garmin.com/en-US/legal/waterrating-definitions/

My understanding is that before the advent of dive computers, dive watches were used for diving so that you could time your ascents and so forth. That's what lead to the development of the (quite strict) ISO 6425 standard.

    they seem like mostly a fashion / status symbol kind of thing

Yeah, this is my understanding from talking to diver friends. As a guy who likes watches yes, they're crazy popular in the watch community.

They've become kind of a genre unto themselves. Citizen, Seiko, and others make affordable ISO 6425 models. I own a few, didn't pay more than $150 for any of them.

Though, I dunno about the "status symbol" aspect. For $30 you can get a diver-style watch that looks like an actual ISO 6425 watch unless you know specifically what you're looking for. So I don't think anybody would be impressed by them. But they are fun to own and rather handsome, if you like that kind of thing.

> If Wikipedia can be trusted here, movement doesn't increase water pressure as much as we think: https://en.wikipedia.org/wiki/Diving_watch#Water_resistance

I stand corrected, but the same article also states:

“Water Resistant 100 m […] NOT suitable for diving.”

“Water Resistant 200 m [.] Suitable for skin diving only. NOT suitable for scuba diving.”

And, IIUC, WR100 is exactly what the Apple Watch Ultra has.

Yeah, totally agree. That's why it's kind of weird/concerning that they conform to the much more lenient standard.

I've been scuba diving / freediving for more than a decade and very, very few divers ever go below 40m. The amount of training, equipment, risk, annoyance, time, ..., associated with 40m+ scuba diving just isn't worth it. Similarly, not a lot of people are capable of freediving to 40m+, and just a handful of people can freedive anywhere near 100m (without mechanical assistance). It would be very disappointing if a watch rated to 100m couldn't accommodate recreational scuba/freediving to 40m.

Sure but this rating is the same for Garmin or Suunto dive computers. EN13319, 100M water resistant. It’s industry standard.