During this most recent stay in Cartagena, I traveled to two nearby beaches accessible by taxi - Boquilla and Manzanilla - and thought it might be helpful to offer a brief report on each.
Boquilla is north of the airport. It's where the new Morros development is located. It was a $20,000 COP taxi ride from Bocagrande. Our driver took us past the new developments through a local neighborhood. We wound up on the beach in front of a restaurant named La Caracolin (not sure about the spelling). The beach was very sparsely populated on a sunny Saturday afternoon. It was really tranquil compared to Cartagena, with just a small handful of peddlers (vendedors). The beach and water were just like Cartagena - grayish sand and murky water. I would like to think the water here is somewhat cleaner here due to the distance from Cartagena (but don't know if that's true). We had a great meal from the restaurant, cazuela des mariscos with langosta (lobster) for $45,000 COP. Everything was very good and the prices more than fair ($8,000 COP for a canopy and chairs). Service was friendly.
Manzanilla is quite a bit farther north. We were quoted a taxi fare of $40,000, but eventually paid $30,000 each way for the round trip. It's a reasonable fare given the distance involved. The beach was moderately crowded on a Sunday afternoon. The sand was a little cleaner than Cartagena's, but the water was still murky. It stays shallow for quite a ways out, great for kids to play in, but a long ways to walk if you want to get your hair wet. There was a strong southward current once we got in deeper water - I wouldn't be surprised if it pulls people out from time to time. Again, very few vendedors. The restaurant was a total rip off though. They basically tried to triple all the prices on us. A lengthy argument ensued when we asked for la cuenta, with both sides threatening to call the police. We wound up paying probably a 50% premium over what locals would have been charged.
On the whole, we preferred Boquilla. I would definitely go there again. Much more pleasant than Cartagena, but without the added complications of catching a boat to Playa Blanca or the Rosarios. Manzanilla really didn't seem worth the extra distance or taxi fare.Edited: 12:09 am, February 18, 2013