My and my friend both followed the "Open Water PADI" course for three days and enjoyed it a lot. Our instructor (Mauricio) was super, he was very professional, helpful and gave us a confident feeling while diving. The course is offered at a very good rate compared to other locations on our planet.
We didn't expect too much of the Coral and the fish having done numerous snorkling over beautiful Coral in Panama amongst others, but we were positively surprised. Other people reported having seen sea turtles as well, while we had seen squid, octopus, lobsters, murenes and many other spiecies of fish.
We slept for one night at the School's premises within Tayrona, which we enjoyed a lot as it was really relaxed (literally nothing to do here but snorkling and relaxing). I read someone reporting that they may have been robbed here, but I lost my Phone here which the guy immediately came bringing back to me, we felt safe here! Do note that it is very basic, which we didn't actually mind.
Anyway! I would advice anyone to follow the course here, and specifically to ask to follow it with Mauricio who is an excellent instructor and native English speaker!