The way to reach Alcantara from Sao Luis is by boat. The crossing takes about an hour. A large motorboat, the Diamantina, leaves twice a day (at 7am and 9:30am) from the Terminal Hidroviario on Avenida Beira Mar in Praia Grande. The price was 10 R$ (January 2008).
The timetable is subject to change, however, depending on the tide. It is good idea to book your ticket the day before and check the departure time. It may even happen that your boat departs (or arrives) from another location; in this case, bus transportation between the two places is provided.
There's a fine view of ruins and the houses of the town as you arrive, strung out along a headland, the skyline dominated by imperial palms; you face a short walk uphill after you disembark.
The cross is quite scenic, you will see all over the coast of Praia Grande and the Canal Entrance right in the mouth of the Bay at Ponta da Areia, the place where the first french colonizers set foot when they first reached our land.
It can be very shaky, so if you go sea sick so easy, better take some medicines...
The boat is commanded by very experienced sailors who also go overseas to fish..and the boat has nice paintings all around, all made by locals!
It will be really a great expereince for you once you decide to visit Maranhão!
At Praia Grande Port close to the city centre of São Luis you can take a morning boat which takes you to Alcantara, it leaves every morning (before 8:00am) when the tyde is high and the same boar returns from Alcantara around 5 in the afternoon, you should pay attention not to miss the returnign boat if you just want to spend one day in Alcantara, otherwise you will have to cross Marcos bay by Catamaran and the sea is WAAAAAAAAAAAAAY ROUGH, and it might not be so pleasant for your traveller's spirit.
GOING THERE BY BOAT IS HIGHLY RECOMMENDED, it is safer.
I will write more about Catamaran, but it should not be a good choice if you wish a confortable trip, Catamaran is for brave sailors!!!!
Morning boats from Sao Luis return in the afternoon. Try to find a boat in good condition.
Mine, "Badevento", got fire while sailing, the day bef. my return so I had to retutn by catamaran, a very wet and funny experience!