Aren't iPads waterproof already.
?rel=0
Classic, even if you don't understand German
Just use a waterproof map pocket with the roll up ends. Same principle as a dry bag. Mine came from Whitworths at about $30 and it works well.
If you are looking for a good holder for your Ipad I've just purchased the one in the photo from www.arkon.com.au $50 delivered.
Seems quite solid with 360 degree rotation and will accept the Ipad within a waterproof bag. You can remove the main cradle and just leave fixing bracket on the boat.