The tune and chorus of Maggie May appear to derive from Benjamin Hanby's Darling Nelly Gray, composed in 1856. Transportation from Britain ended in 1868 although it had become unusual several years earlier.

So unless:
   a) Hanby's song was based on an older sea song; or
   b) Maggie May was composed much later with the intention of sounding like an old song;

there could be a strong possibility of the song originating between 1856 and 1868.

What was the average wage of a merchant seaman in those days? £2-10-0, £3-10-0, £4-10-0, or none of the above?