If its the same version as ps2, I know there is a sea plain by the film studio, face the gates and turn left, its next to the jetti. If its vice city stories it will be a jetski instead.
How To Get An Airplane In GTA Vice City For PSP?
Go to the first island.go to the air port or ocean.in air port there is a plane at the top of the runway intersecting the 2 paralell runways.in the ocean at your safe house by the ocean go strait down the road take a left go down the road there should be a dock. There should be a water plane there.but you can only fly them if you have unlocked th 2nd island.