I've seen this (and other) statements a lot in cases like this, and they've always slightly bothered me. I think it's because this phrase in English is
meant to imply a guaranteed negative outcome- or at least it facilitates the reader towards that conclusion once they think about it. That is- we use it
specifically in cases where someone is about to do something that is, without likely doubt, unwise.
So- here's an example filled out that makes sense: "Just because you can (touch that hot stove), it doesn't mean you should."
But if we fill it out in the case of a watch with water resistance: "Just because you can (take a watch with 50 meters (or more) of water resistance that the manufacturer states is capable of withstanding submersion to that depth), doesn't mean you should," it makes a lot less sense. This isn't a situation in which there is a guaranteed negative outcome. Yes; the water resistance can fail.
Yes, that can be expensive. But it's definitely not a guaranteed thing. The reality is, the risk is statistically miniscule, especially on a watch that is regularly maintained.
I'm not judging your or anyone else's decision on how you wear your watches. Ultimately, each person should weigh the potential risk in any situation, but they should do so from an accurately informed position.
I'll explain my attitude toward wearing expensive watches in the water further. You likely won't agree, but that's OK.
To me it's about calculated risk. What is the upside to taking my >$5,000 watch into the water and what is the
possible downside?
Upside - I get to swim, or snorkel, or dive with my expensive watch. But how much of an upside is that? Is my expensive watch
needed in the water? No. Would I be prevented from swimming, snorkeling, or diving if I didn't wear my expensive watch? No. Could I skip wearing a watch altogether when in the water? Yes. And if the answer to that last question was no, could I wear a much cheaper watch in the water to be able to time things or tell the time? Yes--you could wear a Timex Ironman or Casio G-Shock or one of hundreds of different "cheap" watches.
Downside - The watch company tells you that your watch is safe to go swimming, or snorkeling, or diving. So you should be good to go, right? Probably, but even the "probably" depends on conditions you don't always know with 100% certainty. Are your crowns fully and properly pushed in and/or screwed down? Are your gaskets up to date to be working in prime condition? When was the last time you had your watch water pressure tested? Has your watch taken some unnoticed damage that would compromise its water resistance? As Archer says, a watch is water resistant until it's not.
Why would you take the
chance when the downside is a flooded case that requires an immediate service? Or even worse, a small leak that rusts parts inside the movement because it was not obvious enough to know that the water resistance had been compromised. And the water sits in the movement for days or weeks because you didn't realize it? If you have a water intrusion event you are looking at hundreds of dollars in service costs at best--and a total loss of your watch at worst.
If the watch was
required in the water, let's say it's 1965 and you are a professional diver, I can see the upside being high enough to offset the risk. But today the upside does not outweigh the risk. Omegas, Rolexes, Breitlings, etc. are too damn expensive to risk given that the upside is nothing more than "Weeeee!!!! I'm wearing my super expensive watch in the water!!!!"
So to me, while you can wear a super expensive watch in the water, you shouldn't. And in my case, I don't. But to be totally honest, I have done it in the past. But it's not something I do anymore because it's not worth it.
*There is one condition where I think it might be worth it to wear a watch in the water. And that would be if you were on vacation with a really nice Omega or Rolex, etc., and you feared taking it off while swimming because it could be stolen (either on the beach or from your hotel room). Then I could see the upside (preventing the possible theft of your watch) outweighing the downside (a water intrusion event).
Ultimately, it's your watch and you are free to wear it wherever you want and to do whatever you want to it. But as for me, I'm not taking my Omegas or Rolexes into the water.