Gustavo Rojas Pinilla International Airport is right in the town center and the runway ends at the beach. You must buy a tourist card (COP$44,000) on the mainland before boarding your San Andrés–bound flight, but these are sold at the gate and there's no chance you'll be allowed to board without one. Airlines that connect to San Andrés include Avianca, LATAM and Copa, and cities with direct flights here include Bogotá, Barranquilla, Cali, Cartagena, Medellín and Panama City.
Conocemos Navegando provides pricey catamaran services between San Andrés and Providencia daily, except Tuesday, in both directions. Services depart from Muelle Toninos at 8am and return at 2:30pm the same day. You're supposed to arrive a full hour and a half before departure for registration. The journey takes three hours, and can be extremely rough. It's possible to visit Providencia on a day trip with this service, but it would be very rushed. Seats can often sell out, even in low season, so it's important to book ahead.