Crew is singular, the plural is .. crews.
Can one use crew to apply to a single person? I'm not sure. I am reasonably confident you can use the phrase 'one crew member' though.
Unless I'm getting too old, 'crews' would apply to two or more separate crews of separate ships.
Crew is both singular and plural and refers to whom ever is driving or serving drinks on a single ship, whether that be one or ten people. Of course I would imagine if it is one person in question they would probably not be serving drinks?