I thought the coating wasn't so much for the crew to breathe under water but also for the ship to be able to make the journey to fishmen island.otherwise they would've ended up at marjorea(sp?). The whole reason for the coating was to avoid that so I just assumed all ships under the sea needed coating to stay under. If the fishmen don't need coating on their ships what is so different about their vessels that allows them to avoid that process. B/c it was stated that the underwater pressure would break the 1000 sunny go.