Why would you want to? Don't get me wrong; I like gadgets too, but I can be away from my phone long enough to take a shower. Usually "water resistant" means it can handle a little rain but it's not meant to be totally soaked or immersed. As far as I know there's no such thing as a phone that you can take with you in a swimming pool or that sort of thing. It wouldn't be very practical anyway. You can't talk underwater unless you have a full-face diving mask with a microphone and transmitter built in. That's professional gear, not the sort of thing the average person needs.
I'd keep the phone out of the shower, the pool, or the bathtub. It's not worth the risk.