The ships always have to have enough people on board that they could pull out of port with no notice in an emergency. The different watches are to keep the ship 24/7. Obviously, it's different underway than in port, but the same basic idea applies.
On duty days, you stay onboard for 24 hours and stand whatever watches you have that day. If its not your duty day, you're free to go as soon as the work day is over.
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