The Optio WP can be used underwater... it is not merely resistant. It is a nice compliment to my digital SLR. The propritary battery is a nuisance I'll admit but to get such a small form factor....
What can you really expect of the videos anyway?
As for the 1/2 stop... the Optio WP has no external moving parts which is how it can be truely waterproof. The lack of multiple moving lenses accounts for this f-stop difference. Again, as with the small size, something must be given up. I can forego some features to be able to take a good camera into a disaster zone while on a deployment without worrying about rain and spray and sweat ruining a couple of hundred dollars of camera.