Up a flight of stairs above the Juan Valdez on the Plaza de San Diego, inside the walls. They have been diving Cartagena for thirty years, which in a city where dive shops come and go with the cruise season is the number worth knowing.
Courses from first bubbles to rescue, fun dives on the reefs off Barú and the Rosario islands, and two things most operators do not do: adapted diving for divers with disabilities, and coral restoration work you can join.
They publish no telephone number at all. The contact page gives one line and labels it "Solo WhatsApp" — messages only, which for a dive shop with a boat out most mornings is honest rather than evasive.