Most of the waterproof boxes I have seen which are actually waterproof are also pretty heavy. Most boxes will however float at least long enough to be retrieved where possible.
Merril makes a box of an all foam construction which is held closed with magnets in the corners. These are light as a feather and float like corks. In fact I doubt they will ever sink.
If you flies do get wet, open the boxes and let them dry. A hair dryer used carefully can speed the process as will placing them where they can be heated slightly by a light bulb, just don't get them too close or you will surely melt something. Once dried they should be good to go!
Tight lines,
George