There are two ways I did it. One is expensive and might not work.
1st... go to the coit tower and go up as high as you can go. look over to the rock and just jump and hold down your jetpack. (warning!) you need to have the jetpack upgraded to at least level two.
2nd... this is expensive. Go over to the prison (where you killed bongwater.) either jump on the rail, or pk a car up there. Jump on your chosen platform and jetpack 4 all your worth. (warning!) you need level three jetpack to do this! hope this helps!