If you are just looking for a waterproof casing, then go with the OverBoard or Aquapac. The reviews for these cases are pretty good. All of the touch functions and such are still functionable with the casing on top of it. They don't really add protection from dry land drops through (think of a ziplock bag). If you want a case that's only made for waterproof then go with one of these brands. They run around $35 or so.
You might be familiar with Lifeproof cases. It is more of a case which can be kept on the iPad at all times. It is waterproof and also protects from dry land drops. So if you want a case that's waterproof and want to keep it on all the time then go with this one. The price for a Lifeproof case is pretty much double at just below $80.
Before you decide on either case you should read more about it in this article:
http://supremeskins.com/top-3-waterproof-ipad-case/
