I've owned a Z3 compact for a year. Best phone I've ever had. Not because of the waterproofing but due to the awesome battery life and small (in today's terms) size. But that's just by my criteria.
As other owners of the line have noted above: yes I do use it while having a bath, and I casually wash it off under tap water. Never taken it to a pool, though it has been submerged in the bathtub (though the depth would be something like 10cm). Would I try to take an underwater photo? Yes, I wouldn't think twice about it. It is possible to access the camera and shoot merely through tapping the screen. Would I use the fast-shoot button though? Probably not, I'd avoid any mechanical moving part underwater.
The USB flap is the most annoying thing on the phone. I _NEVER_ use it. I bought the magnetic cable that sticks to the side of the phone where the two charging pins are in my first week of having the phone. There is a second flap where the mini-SD card plugs, which I assume is still there, though I doubt anyone would need to use it often (as opposed to the USB port which is used for charging). I'm 100% certain that if you open this port daily it will gradually wear (it's mad of rubber I think) and lose its effectiveness.
In short, I think that the phone will do everything that is claimed in the ad, as long as it's used properly (don't forget the flap open) and it is new (flap won't help after X number of open/close cycles). So, the catch is that normal wear and tear (i.e. using the USB port flap every other day for charging) will probably break its waterproof ability.
It sucks, but they seem to have fixed it in the Z5 compact (thinking of buying it) which has a flap-less USB port that is waterproof. And even the current model has ways around it (the magnetic charging cable I mentioned, or a dock, combined with transferring files via WLAN rather than attaching it to your computer with USB).