Ferries to Sidi Yousef (Kerkennah) leave from the port, just a five minute walk from the centre of town. Make your way to the waterfront with its cafes, continue up the road, past a more traditional smoke-filled cafe, cross the railway line and the ferry terminal is in front of you.
There seem to be quite a few crossings every day, at least 10 all year round. The journey takes just over an hour and is one of the best travel bargains around, costing a meagre 0.65TD (April 2007: GB£1 = 2.5TD)
The frequency of the crossings and the ease of getting around the Kerkennah Islands means that you can make a day trip to the islands if you get up early enough. Shared taxis meet the ferry in Sidi Yousef and serve all the main villages...you could try to rent a bike for a day from one of the hotels in Sidi Frej or Remla, or just spend the day walking.
No need to book tickets in either direction...just turn up, buy it and board. Cars can also cross, although tickets will obviously cost a bit more and you may need to reserve in advance as the boats aren't huge.
Anyway, the ferry is an enjoyable experience, whether in good weather, when the upper deck has a party atmosphere about it, or in bad weather, when nither the islands or the mainland are visible and you feel like you're going on a voyage to some remote outpost.
Look out for Sfax's huge phosphate "mountain" south of the city, and closer to Kerkennah, the shallow water fishing traps which begin to appear about 15 minutes from Sidi Yousef.
"Why don't you take the train?" suggested the man at reception. Yes, I thought, why don't I take the train? I had yet to experience a Tunisian train, and was assured it was more comfortable than louages. The advantages were that I would know when exactly I would leave, and roughly when I would arrive, and I would be able to see a few sights on the way (El Jem, Soussa, etc...). All started off well, with a ticket containing a seat number handed to me with little fuss...1pm, I was told, so I went off back to Sfax medina for a last coffee at Cafe Diwan.
I thought I'd be early, just in case it was similar to a Sudanese train, where there are no seats and everybody fights for a bit of floor space. But surely this couldn't be like that, in Tunisia? "Oh no", said the little man at reception, "our trains are excellent, no sitting on floor, haha, oh no".
I didn't know whether to laugh or cry when I boarded the train an hour before departure. Every seat contained one and a half bodies, my reserved seat counted for nothing, and most of the floor space had already been bagged. I managed to park my bum in the doorway between a group of students, and we looked on fascinated as yet more passengers threw themselves in the door and exclaimed "no seats!" and still pushed their way through anyway.
It was punctual though, trundling out of Sfax a little after 1. We might have passed the amphitheatre of El Jem, we might have afforded amazing views over the coast near Soussa, but I couldn't tell you....my view was of the toilet door and somebody's backside. The situation was not helped by the arrival of a refreshments trolley too big for the already overcrowded aisle, and just when you got semi comfortable, one of the seated passengers would come in and make us all stand up because he wanted to smoke a cigarette. Finally got a seat after Soussa, but many others were not so lucky. Was this a special day? or are all trains like that here?
Still...it got me to Tunis in 4 hours, so i can't really complain...much.
Sfax is a major city, so louages leave all day long for all over the country. For an idea of price, I took a louage from Le Kef to Sfax, which took 4 hours and cost 12TD. South to Gabes, a journey taking 2 hours, cost 6TD. Mahdia was around two hours away to the north, but I haven't a clue how much that cost as it was many years ago!
Sfax's louage station is unusual in that you are required to buy a ticket from an office before climbing aboard. I suppose this is a good system, as you can be sure you will not be overcharged. It also means that if you are travelling at night and there are not enough passengers to fill a louage, you will still travel and won't have to contribute for the empty seats.