The argument of waterproof vs. water resistance does not hold water any more. Sorry for the pun.
Look at the IP ratings and IP67 does in fact allow submersion.
One has to remember, however, these are static tests. Real life is hardly static. If a phone is dropped in a tub with a depth of 15cm from a height of 1 meters, the initial plunge is going to have dynamic pressure higher than the rating. It's OK just a fuse rated at 20A won't blow when a motor that pulls 35A for an instant while starting.
Swimming, OTOH, will cause all sorts of pressure variances exceeding on multiple planes. Most of the time this "exceeding the limit" is harmless but there's never a guarantee that you won't experience a failure.
Testing the water proofness of our devices isn't routine. It's not a one shot feature say like an automobile airbag, flashbulb (for those old enough to remember those!), or dry chemical fire extinguisher. But it's not something you should do like test your smoke detectors or take a firearm to the range either. It falls somewhere in between. For those aw5hit moments like a beer getting knocked over by the family pet or a drop in the toilet. Yuck!
Even so, there are places for fluids to get in and even though it may not reach the circuit board or switch, it will change the device. This is cumulative damage. Leisurely dunking the phone isn't probably something you want to do outside of those folks on youtube (you know who you are!) that count on views to pay for their destructive repertoire!
It's peace of mind knowing if you leave your phone next to the sink and it falls in and gets wet you don't have to scramble pronto looking for a baggie and cupful of rice.