I guess it depends on your interpretation of the song, but I'm not certain that it's necessary for the plot of the song to specify that it is a ferry as opposed to any other kind of voyage that you are paying for; the key, in my mind, is the idea of paying the person responsible for your safe passage before said goal has been achieved, for fear that they will not deliver on their part of the bargain.

I didn't say it was necessary for the plot of the song, just that it might be a better translation of ferryman. I wasn't making any corrections, just a couple of suggestions.

-- 
SuStel
http://trimboli.name