"push down whilst skimming along the water and when it
takes off, push up and it will level out. The problem is, if it lands
on land, you're buggered as it won't take off on land or even move,
so be sure to land only in the sea. Now, the best place to start is
Viceport, then just follow them to Little Havana, then to the airpot
and then a back to downtown."
FROM GAMEFAQS.
its not really that difficult a mission, just keep trying, its easier with a gamepad
than the keyboard