Almost all water around the Keys out past 3 nm is inside the Florida Keys National Marine Sanctuary. The sanctuary bans dumping 'any material' from inside its boundary, and ash scattering is not one of the exceptions, so you need a free-to-apply NOAA sanctuary cremation permit (online form at floridakeys.noaa.gov/permits/special-use.html). NOAA only permits ceremonies at least 3 nm from shore, in federal sanctuary waters; only ashes and flowers - no urns, boxes or plaques. Stay out of Sanctuary Preservation Areas and Ecological Reserves, where only engine cooling water and exhaust may be discharged. Use the Atlantic side: 3 nm off Islamorada is near or past the reef line (estimate - check chart for Alligator Reef and other preservation areas). Florida Bay (Gulf side) is shallow, inside the sanctuary/Everglades area and not covered by NOAA's 3 nm permit practice.