There's no such thing as a "water proof" watch. Only certain levels of resistance. The G Watch is rated to withstand submersion into a maximum of 1 meter of water for up to 30 minutes (or equivalent pressure). Taking a shower could potentially exceed that (depending on your water pressure), as could swimming.
Agreeing with spiderflash, I also wouldn't worry about it while raining, washing my hands, going to Splash Mountain, or even washing the watch in the average sink. But I definitely wouldn't be bringing it with me into the shower, ocean, or swimming. Beach/sand would be fine, as the watch is dust proof. So long as you don't mind the sand scratching it to hell. I wouldn't bring it to the beach either.
If I'm doing one of those other activities...that's what my Timex Ironman is for!