Back in 197.... forget the year, Bristow had an S61N on a medevac ditch in the North Sea after it lost a pocket or more from a main blade. The sea conditions were such that it turned over. In those days we didn't wear survival suits, just the UVIC 'floater' neoprene jacket. The pilot did a good job in hanging on to the casualty on the upturned hull and stopping himself being washed off by hanging on to the undercarriage. He was unconscious when winched into the rescue helicopter and actually lost his licence as a result of the accident. After that floats were fitted and the sea anchor was mounted such that it could be released from the cockpit, so I'd say that external floats are not that uncommon on boat-hulled helicopters.