I didn't know there were flats around Cosumel for bones. I've also be down to Boca and Pez Maya and caught bones and permit (although small). I've used the crab fly with great success and have had to buy them as I don't tie. Crab flies of any size do just fine.
If you go down that way don't miss Akumal and the ruins there....you drive right past it on your drive.
