For your own safety diving is not something you want to go cheap on, beware of 2 for 1 deals and suspiciously low prices or garage looking dive shops.
I will only vouch for two dive shops in San Andres and those are:
Blue Life Dive
Blue Life Dive is run my very good childhood friend Fabian Garcia and his family, if you google Fabian you'll see that he's one of the top divers in Colombia. In my opinion they have the best run operation in all San Andres, they have the best most complete diving boat in the island, we recommend this dive school for more experienced divers, however they are about 15% above market price.
They also hold a great location downtown at the Sunrise Beach hotel; which gives them access to the pool for the Discover Scuba and the Refresher courses and also gives them full access to the hotel's pier, which means that all of their dives are done from the boat. At the time of this writing they have 2 boats which they use depending on how much people they have on a given day.
You can check Blue Life's rates here is worth noticing that they are the only 5 Star dive shop in the island, you can check this at the Padi website.
Scuba San Andres
Scuba San Andres, is run by my uncle Mo Gomez, the fact that we are family doesn't cloud my ethic of recommending them, because I know first hand that they offer incredible service as experience by more than 500 of our guests.
Scuba San Andres is located on the West side of the island, inside Playa Tranquilo hotel right in front of one of the best dive spots; as their location is far, all of their courses and fun dives include transportation from/to the hostel.