No. The defenders aren't waterproof at all and only a little bit water resistant. If you're going to be around water with your phone a lot the Otterbox Armor Series or a LifeProof. Seeing as Lifeproof have been in the business of waterproof phone cases a while, I'd tend to go with them for waterproof / dust proof however that statement isn't based in any empirical evidence.
I have a defender on my iPhone 5, it's a brilliant case and it's heaps tough. I'd recommend it to anyone.