There are several ferries (actually most are just fast open boats with twin outboards). Whether you can make it depends on your flight arrival time. The last boats go to San Pedro around 3.
I personally think the Caye Caulker Water Taxi Association boats are the best. They are wel organized and leave from the Marine Terminal, which is a safe place to wait, and there's a waiting room and a couple of museums in the building to visit while you wait.
While you can make reservations by phone for the ferries/water taxis, as a practical matter it is more first-come, first-serve.
Fares are US$12.50 one-way. Plus you'll need to take a taxi into town, which is US$17.50 (for the taxi, not per person). Tell the driver you want to go to the Marine Terminal at the Swing Bridge.
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