Cartagena sells two things: the walled city, best seen on foot with somebody who can tell you what you are looking at, and the water. The Rosario islands are an hour out by fast boat, and the diving on the reef is the reason the dive shops are here.
Prices for the island boats vary more than anything else in this guide, because what you are paying for — the boat, the lunch, the number of stops, whether the park fee is included — varies too.