Defense for what?
It depends on the vessel. A 24 hour liveaboard is required one licensed Master, one licensed Mate, and 2 deckhands. The deckhand is a position, just like Master, and requires some training to be competent. Documented training performed by the Master is sufficient to satisfy the requirements of the training. If not a Master, Mate or Deckhand, the other "crew" may be stewards, which have no duty to the vessel, or may also be deckhands, and be counted as deckhands.
My stewards were trained and in service to the vessel. One steward had the responsibility to ensure everyone was evacuated from the inside of the vessel during fire, flooding, or casualties. The other steward was responsible to hand out life jackets and ensure that they were properly worn. We ran fire drills every week and abandon ship drills every first morning just before the second dive.
Everyone participated. When we had a CG safety inspection one year, it was just me, my wife (a licensed Master) and a stewardess. CG pulled her aside and quietly asked her if she knew how to start the fire pump. She said yes. You could start the fire pump from a number of places, so she reached up, started the fire pump, faked out a hose, charged it, flowed it over the side, and asked "Now what?". So when we got underway for man overboard drills, they made her be the deckhand to recover Oscar. Oscar went over the side, lost his shorts, she grabbed a radio, guided me around, when we came alongside Oscar, she had a life jacket and life ring with rope, went in and grabbed Oscar. She weighs 95 lbs, Oscar weighs 85, she had a hard time with him on the ladder, but she got him to the swim step.
Her job was to be kind to the customers, serve them morning coffee and evening wine. She made one hell of a deckhand.