The Island of Roatan
The island of Roatan in Honduras is one the best locations to work on a scuba instructor certification in the Caribbean. Honduras is cheap in general, and savings in excess of $1000 can be realized by taking a course or internship there instead of in areas like the United States or other more popular diving islands. An internship with a diving company on the island, all inclusive minus drinking, costs only $2000 USD. That is a two month vacation that includes everything, and at the end you receive your instructor certification. There are other beautiful tropical islands for scuba diving in the Caribbean, but none are cheaper.
The easiest way to reach the island is by plane, though it can also be reached easily by boat if need be. It is a popular destination along Central American travel routes for divers wanting to avoid crowds. The summer months are more crowded than usual, so going in the fall around September will offer a very tranquil experience. If you are looking for a scuba certification for cheap, you can do much worse than Roatan.