"Mariners" may be a more suitable spelling for the title in terms of searching for the song. But "mar'ners" is surely appropriate for writing out the song(as is done in the Penguin Book ).Mmario's tune clearly has one note for the syllable "mar'", and not two for "mari". In practise the truth is somewhere in between, the second syllable being very minimal, more of a roll on the "r"