Sure, this video must be fake, but the sound is so clear I find hard to believe that it is inside a case. Then again, I find even harder to believe that any iPhone can survive this.
There's only one way to know the truth: Get your iPhone 3GS to the pool today, submerge it for a few seconds, and tell us. Come on. You know you want to do it. Send me your test video to firstname.lastname@example.org.