By cargo boat
Cargo boats don't have a fixed schedule and we don't really know who to contact, but is you are staying in San Andres for a while and have the time to ask around, this might worth a shot the people who have done it told me they paid $50.000 COP per route.
From what I've heard is up to the captain of the boat to allow you to go with them as is technically illegal to transport people that are not crew members in these types of boats. From a pure statistical point of view We have noticed that women have a higher success rate than guys, so let your lady friend do the talking.
Your best bet is to go to the dock (5 minutes away from our hostel) and ask the captains directly, please be aware that to enter the dock for safety reasons you must be wearing pants and fully covered shoes, otherwise they won't let you in pass the main gate.