Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
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.

this looks sturdy. for the price, it had better work at the submersion rating...
 
They're coming... check this site out for more info.

http://www.lifeproof.com/

Seeing as though the Otterbox Defender for Ipad 2 is getting pretty bad reviews, these folks could really clean up if they get their product to market sooner rather than later.

Looking at how long its taken them to get their version of this case out for the iphone, still in pre-orders, I'm not holding my breath.

They told me this summer, I would bet more likely in 2012!
 
Overboard

only one ive seen is the overboard case .

its not the most stylish thing ive seen , but it does the job-apparently.

its on there website over-board.co.uk for around £30.00
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.