The technical meaning of "waterproof" or "water resistant" without qualification is 3ATM or 100 feet at sea level for 30 minutes. It sounds impressive but it is good only for hand-washing under a slow-running tap and light rain without any wind driving it. A fully open tap with a good head of water in the crawlspace above it -- which is your average bathroom or kitchen washbasin tap -- will defeat it, as will heavy or wind-driven rain.
There's an annual service cycle for this standard and every other ATM rating, and for higher ratings like 20ATM or 670ft an actual pressure test at every annual service.
Comprehension is not furthered by Chinese manufacturers throwing imaginary ATM ratings around like confetti. Since people have discovered which Chinese makers not to believe, the Chinese have started using IP67 and IP68 ratings to confuse the issue, which in their hands are even more worthless. Most worthless of all is a rating the Chinese invented: "5ATM life water resistance" which means less than 3ATM. or "Don't try washing your car while wearing this one!"
A waterproof instrument like a heart monitor or handlebar computer, to have a credible 100ATM rating (long and within living memory a professional diver's rating, today that is 20ATM), should have all controls and ports threaded and screwed down, with O-rings compressed under the head, and all threads and O-rings dipped in Silicone grease annually and at all battery changes.
For 20ATM you need to ask about the precision of the thread cutting and matching as well.
Uprating to 30 ATM or 1000ft in my opinion requires circular half-round recess cutting for O-ring positioning and precision, which is expensive engineering and forbiddingly expensive by the time it reaches a retail sales counter. The point of such a watch or other device isn't to dive to a thousand feet but to have better waterproofing at the limiting depth of lesser watches or diving equipment or the human body.
30ATM is overkill for any conditions a bicycle or bicyclist is likely to meet, but I think a true 20ATM device could be justified on tour because you can swim with it, and you may not want to leave it on the bike; it should, if in good nick, also resist, say, monsoon events long enough to find shelter.
Note that all these ratings presume cold water. Hot water or even a steamy environment will certainly defeat any rating under 10ATM and shorten the rebuild and test interval even on the better ratings.
BTW, don't put an iPhone (or any other phone) in even a sweaty pants pocket, or you might be refused warranty service and have to buy a new one.
