The ferry terminal on the São Luís waterfront is the departure point for boats to Alcântara across the bay, and also for boat tours of the bay of São Marcos. The terminal building itself is unremarkable, but the quayside around it is lively in the mornings when the Alcântara ferries depart and fishermen bring in their overnight catches.
The ferry to Alcântara runs twice daily and takes about 90 minutes. Alcântara is a colonial ghost town, wealthy in the 17th and 18th centuries, now beautiful in its decay. The crossing itself is also pleasant — the views of the São Luís skyline from the water, and the mangrove-lined shores of the bay, are worth the trip even if you don't want to spend time in Alcântara.