Get an Otterbox - they are designed for diving (I think) and are great for electonics. You can even get some models that are designed for specific pieces of kit and have "buttons" in the right places.
Failing that I use freezer bags - the see through ones with the "ziplock" seal - you can leave a little air in them to make them float as well. Always use them for my mobile phone etc when on the boat - they work fine and are pretty cheap at about 5p each!!!
