Hi Carmen,
Coiba island is a great place for diving, but I don’t think it’s the best environment to learn the sport (I even worked there as instructor for a few months and didn’t like the conditions for the courses).
You will need to do a lot of skills and exercises during the Open Water course and Coiba island is 1 hour boat ride from Santa Catalina, which makes the whole process very tiring, having to go there and back every day.
Even if you stay on the island, the beaches are not the best. You can’t use some of them because there are crocodiles, and the safe ones are affected by the tides.
I would recommend to get certified somewhere with better conditions for the course, near dive sites, short boat rides, calm seas, etc like the Caribbean coast. It doesn’t matter if the diving is not great, because you will be doing skills and exercises most of the time.
Then, once you finish the course, go to Coiba island for some of the best scuba diving in Central America 😉