For boating, the most professional looking solutions I have seen is from a company called Watershed. Their
Grid iPad bag is rated for water submersion up to 300 feet.
Here is a
video of a guy using this case throwing his iPad into the ocean and it kept things dry.
I haven't personally used this case, but what impresses me is these guys have been specializing in waterproof enclosures for other applications for quite some time. So, they aren't just some new company trying to sell an iPad accessory that is really just an overpriced freezer bag.