The cheapest one is by Navionics of all people. I downloaded all the Caribbean charts along with the program for $9.99 for the Iphone works great. I understand it works on the Ipad but since it's made for a smaller screen it will be a bit jagged. I'm sure that a higher definition one will be made for the Ipad. GPSNavX also makes a program but it's about $50.00. As I understand, Faguai also has one.