The best way to get to San Andrés Island is through Bogotá, Medellín, Cali and Barranquilla. These are main cities in Colombia from where different airline flights depart every day, which gives you chances for better prices and schedules. Some airlines offering flights to San Andrés are Copa Airlines, LAN, Viva Colombia and Avianca. According to your travel itinerary, you can carefully plan from which city to depart; the most recommended one is Bogotá because of the increased frequency of flights.
If you have your own boat, you can sail it to get there. Plan your trip and get to San Andrés where you will find marinas for your boat.