Start with the travel time, not the sand
The water near Cartagena gets clearer the further south and west you go, and every beach worth the trip is measured in boat minutes rather than kilometres. That single fact decides most of these choices. A city beach costs you nothing but a walk and gives you cloudy water. The clear Caribbean postcard costs you most of a morning each way, unless you're already sleeping out on the islands.
Here they are in order of how far you have to travel.
Bocagrande: the beach you can walk to
Bocagrande is Cartagena's own beach, on the high-rise peninsula just south of the old city, and the reason to use it is proximity rather than beauty. It is a flat sandy strip with the city directly behind it, which makes it genuinely useful if you have an afternoon spare and no appetite for arranging a boat. The water is the working harbour's water, not the turquoise you've seen in photographs. Treat it as a place to sit for an hour, not the day you flew here for.
Tierra Bomba: the shortest crossing there is
Tierra Bomba is the island directly across the bay, and boats reach it in roughly 10 to 15 minutes from Castillogrande beach, behind the hospital in Bocagrande. That is the shortest crossing to real island water anywhere near the city. Beach clubs on the island, among them Bomba Beach Club, run day passes, which is how most visitors use it. Go if you have one free day and want sea without losing the whole of it to travel; skip it if you are heading to Baru or the Rosarios anyway, because the water out there is better.
Playa Blanca, on Baru: famous, white, and busy
Playa Blanca is the long white sand beach on the west side of Baru Island, and it is the most photographed beach near Cartagena for good reason and the most crowded for the same reason. Day boats from Cartagena arrive mid-morning and leave mid-afternoon, so the beach has a rhythm: hectic from about eleven until three, calm either side. If you can be there early, be there early. We've written a longer and more honest read on it separately, including who will enjoy it and who won't.
Cholon Bay: a bay rather than a beach, and that's the point
Cholon Bay is the sheltered bay on the inner side of Baru where we are, and it is a boat and swimming bay rather than a sand beach. On weekends it fills with boats; midweek it is quiet enough to hear birds across the water. We're on it, and the reason we'd point you here over a beach is the mangroves: you can kayak the channels behind the bay at first light, which is the best hour for birds and the only hour the water is glass. It is a different kind of day from lying on sand, and better company for anyone who gets restless on a towel.
The Rosario Islands: the clearest water, and a national park
The Rosario Islands hold the clearest water near Cartagena, and they sit 45 to 70 minutes out by boat from the city. The archipelago lies within Corales del Rosario National Park, a protected coral area, which is why the snorkelling is in a different class from anything closer in. From a day boat leaving the city you get a few hours there at best. From Baru you're already most of the way, which is the practical argument for sleeping on the island rather than commuting to it.
Where we fit
We're on Baru Island at Bahia de Cholon, and our boat takes 30 minutes to an hour from Cartagena. Door to door from Rafael Nunez International Airport is an hour. The boat has limited capacity, so crossings need reserving in advance rather than arranging on the day; if there's no space, the wait is a long one.
What that buys you is simple arithmetic. A beach day from the city is two boat journeys and a fixed return time. From here the far water is a short ride, and you choose when to come back. Six rooms across three houses, so the bay in front of us is rarely crowded by our own guests.
A note on the order of a trip
If you're doing both the city and the water, do the city first. Ending a quiet stretch on the island with a return to traffic and a flight is the wrong shape for a holiday, and the crossing is short enough that splitting the trip costs you no real time.