As previously mentioned by other responders, the dive operators in Cartagena mostly will dive where there isn't a pollution problem, namely around Isla Baru and Rosario Islands. I went out with (and recommend) Cartagena Divers, a PADI 5-Star operation located in the Walled (Colonial) City; we dived around Isla Baru. True; the diving in general is on par with places on the Mesoamerican Reef, such as Cozumel or Honduras' Bay Islands. But I thought it still was very satisfactory: pretty good viz; warm water temps; healthy aquatic life. (Please read my feed where I describe my trip, which was quite recent: June of 2018.)
I also have dived from Taganga-Santa Marta. The same can be said: pretty good (not excellent) viz; the water seemed slightly cooler, but still Caribbean temps; healthy marine life, in my observation. The dive operators -as well as everything else- are MUCH less expensive than in Cartagena, and Taganga has many from which to choose. I went with Oceano Scuba, and I would recommend them. I also did hear that Poseidon was a good operator, as well. I went diving in Taganga in 2016; I look forward to returning, at some point.
If you have a tighter budget, Santa Marta definitely is the better choice. Topside, the area also is more laid-back and less a tourist trap than is Cartagena. That being said, Santa Marta is not a beautiful city; and Taganga is something of a dump. Cartagena, meanwhile, is quite touristy and significantly more expensive; but it has a gorgeous colonial section (the "Walled City"), while some other parts are built up and look like South Florida. You choose your preference.
In conclusion, I have enjoyed my scuba experiences in Colombia, both in Cartagena and in Santa Marta-Taganga. Consider what I and other SB users have said. Personally, I do recommend scuba diving in Colombia. I have found the operators (that I have chosen) to be courteous, safety oriented, and professional. The underwater environment is very good. I cannot comment on diving the Pacific side; maybe I'll go there on a future trip.