Here's a car roof box that doubles as a small boat/canoe.
http://uncrate.com/stuff/boatpack/
I wonder how difficult it will be to retrofit an existing car box with a reinforced fiberglass shell? I'd rather do the work myself and spend $400 worth of fiberglass and resin on a custom "boat", than fork over $1,600.
My buddy did these two videos of his car top carrier conversion into a small boat.
http://www.youtube.com/watch?v=FUDDsEC3a9c
http://www.youtube.com/watch?v=w90Uzv3kH74
and part two..
http://www.youtube.com/watch?v=FUDDsEC3a9c
Should be easy. Look into foam sandwich construction. A skin, foam spacer and another skin. Super light and strong. You'll probably have to add in some stiffening ribs to your basic car top carrier shape , usually 1x 1 strips of foam covered in glass and carried on to the base. Or wood strips will do fine. Glue some styrofoam to your shape ti fill everything between the ribs, then cover with glass cloth. Don't use matt, use weaved cloth. Usually on a 45 deg. bias or whatever direction you need the strength. Styrofoam will be eaten by polyester resin so you'll need to use urethane foam. Or use styrofoam (the blue or pink stuff) and epoxy resin. Check first if your resin is going to eat at or even stick to the plastic. Epoxy is your best bet to stick to plastic and most expensive but also won't shrink with age.