As a funeral director, whenever I know a family plans on flying with cremains, we always make sure the urn Is either plastic, wood it cardboard to pass through as carry on. ,All that's needed is a copy of the death certificate and cremation permit. I always stress never put an urn in checked luggage , granted this is a different issue than getting a " burial at sea permit" but after 9/11 you don't want to second guess Airport security. Once you land, who's to say where or what you are going to do with the cremains. There are no laws about traveling with cremains. So what if they "fall" over board,,,, but by all means make sure you get on and off
the plane.