Waterproof clear map cases are great -- they're sortof ziploc baggies made from extra-thick material with a ziploc enclosure -- perfect for surviving just about any brief water exposure (I think they're rated to 3feet, though that might be 3 meters). You can even read in the shower if you wanna
I'll hunt around to see if I can find a link to the map case that I'm currently using...I think I bought it in the kayaking/camping section of Cabela's.....
Edit: Here's a set of cases similar to what I have, good down to 15 ft:
Cabela's
And
here's a store with a great online selection of waterproof map cases. The good ones have reinforced sides and D-rings for attaching in various ways (I use a shoelace looped through the D-rings).
This one's good to 30ft (9x12 or 12x16):