Water damage is water damage -- the phone won't magically tell Apple "I got hurt because I went in deeper than 1m, so don't cover me!"
Why not?
The barometer tells them how many steps you climbed. Water pressure is very linear, and there's no need to be strict about the cutoff: warranty to less than the certified depth for a reasonable time, then reject based on acceptable claim rates. Customers are happy, Apple scores points with an industry first and even gets a bunch of much better real-world data to use for improvements, cost cutting and postlaunch QA. Win-win.
You're talking about $500+ devices here, Apple isn't going to eat the cost of every joker who goes deep-sea diving with their phones beyond spec.
There aren't that many jokers like that, and a cheap (when mass produced) MEMS burst-disc valve can tell you if there was substantial overpressure, a function similar to the water damage indicator. Or go with the barometer. Then reject out of spec damage based on acceptable claim rates, again. Stick something in the water indicator compound to flag ion levels in the water to do the same for salts, acids and the like. Then, just for giggles and profit, include a reservation against repeat claims on similar damages, series of claims on different damages, and intentional tests of durability (i.e. good faith claims only).
Or, you know, fix it when necessary with a "subject to change" clause from the get-go and start off with just the claims rate limiting. I doubt they'll need to clamp down even once if their QA is decent.
I still don't want to take my phone outside in the rain but hell, I admit I'm damn comforted by the idea that if I did, I should be fine.
If there were some reasonable indication of what it can take, i.e. what they will cover if it doesn't, I'd be comforted. Right now, I'm just seeing an increase from one unknown quantity to another. If only we had some way to make it clear what my expectation should be, like, maybe a certification level. That would be nice. </s> And a selling point of note.
Where I live, the reasonable expectations of the consumers are rule of law in effect, and the burden of proof when rejecting a claim is on Apple. I don't have this phone, preferring the size of the SE, but let's say I did. So I take it for a dive and it dies. I hand it to Apple, say it was out in the rain, cross my heart and all that. Per your argument, they can't tell the difference, so they just have to pay up. (If they did have a barometer in the phone, as they do, and used it, as they apparently don't, they could just show that, or point out that the GPS indicated I lost the signal at sea after it progressively weakened as I descended below sea level.)
And yet, despite these generous consumer protection laws, there aren't a ton of claims being made... and Apple generally just pays up with a smile, leans back and "watches the zeroes pile up behind a meaningless figure", one that is bolstered by this confidence in proper customer service and warranty coverage.
This is one of those times where they should ask "why" and "why not" and just go with "courage", IMO.
I'm not gonna complain that they don't- got no horse in the race- just noting there's a win to be had here.