I think you're better off just getting your phone treated to be waterproof so you can use whatever case you want, or none at all.
Liquipel and Never Wet are two that come to mind. They cover the device with some sort of vapor that dries and leaves a protective film across it.
I've never used it, but it's an interesting concept. If I were still in the Navy, I'd have done it a long time ago.
Sent from CAMACHO, my Verizon G2 (VS980) running PAC ROM 4.3.
The Aphotic Chaotic
LG G2 VS980 (Verizon) [JB 4.3] PAC-man ROM
Nexus 7 [JB 4.2.2] XenonHD