I am a trainer and generally have the announcement say "after leaving the aircraft". When I train in ditchings I have the flight attendant inflate the vest as they are jumping into the pool - not before. Also we train to use overwing exits in a ditching as primary exits as the main exit may be below water level. On a turbo prop aircraft we train to use the overwing exits and not the floor level exits as those exits may be below the water level. On aircraft with slide/rafts generally the aft exit is not used but this depends on the attitude of the aircraft once in the water.