Leaving your iPhone pool or beach side while taking a dip can be a risky move, so a waterproof iPhone case seems like a smart idea. This one even has a special jack for use with waterproof headphones.
The protectively sealed bag design has clear windows so you can still use your iPhone's touchscreen, and take under water happy snaps. I''m not sure that I'd personally want to risk it, but you can take the plunge for 3,280 yen (about $40). [Sanwa via DVICE]