Runs two catamarans, El Sensation and El Splendor, between San Andrés Town's Muelle Toninos and the Muelle Municipal in Santa Isabel, Providencia. Departures are daily at 8am, except Tuesday and Saturday, from San Andrés, though you'll need to be there at 6.30am. The journey takes three hours. The return boat leaves at 2.30pm, but check-in is at 1.30pm.
Buy tickets in advance from the office in San Andrés Town, as the catamaran is often full.