I use a water-proof map bag when at the beach/lake. They're under $30 and will keep your device dry as you float around on an air-mattress. I thought I got my last one from here for about $15, but may be mistaken.
http://www.mec.ca/Products/product_d...34374302888436
I'm sure the REIs and whatnot in the US, and similar sporting/camping stores will carry something similar.
I kept hoping for one of the hard waterproof cases like the kindle have, but never found anything local to test.