Hi,

Have never been aware of a Sfax to Djerba ferry and pretty sure this does not exist. Ferries run between Sfax and the Kerkennah Islands, and for Djerba ferries go from Jorf on the mainland to Ajim on Djerba. Jorf however is pretty much opposite Djerba, so nowhere near Sfax.

You may find this website useful for checking ferry routes and info in Tunisia:

http://www.ferrylines.com/en/ferries/search-results/destination/Tunisia///

I've not used this site for confirming info myself, as when I've used the Djerba ferry, we just turned up. The Jorf-Ajim-Jorf route runs fairly frequently, is quite cheap - if memory serves me well, and you can take a car.