Last night me and my friend did some searching on the internet for it and founfd out where it was. We used the tank cheat and the dodo cheat and flew out from francis international airport. After that take a right behind the mountain where the damn is and you should see a floating island. It is called Carser city and they even give you directions for it on the radio. Its the same city where the start video takes place when they rob the bank. Be careful though you can only land on a few spots, and if you miss you drown. PS there are cars on this islnad.