Methinks you don't know what "waterproof" means. The headphone jack has to be waterproof if unplugged, or the phone itself can't be waterproof. Whether water can get into the area of the jack where it's designed to be allowed doesn't change the fact that it's waterproof *in the areas it is supposed to be*.
Also, fresh water is a poor conductor, so it should not be meaningfully completing a headphone circuit -- at least, if you dropped your phone into just water. Now, if you dropped it in dirty or salt water (which is all I can presume from your complaint), that's an entirely different situation -- and one neither existing waterproof phones with headphone jacks *and* the new iPhone are designed to survive. Check the manual for both and it will specifically tell you not to immerse the phone in salt water.
All I can tell you is that my phone has been immersed in fresh water multiple times, mostly by mistake, but as designed it continued to work just fine, and as soon as the touch-screen was dried, did not show any untoward behavior of any kind -- and that includes complaining about headphones. But even if that really happened to you, one shake of the phone would have been enough to remove the water from the headphone jack and stop the problem.