Instructions
Star Fish Island
Go to a hospital and steal an ambulance. Drive across the Star Fish Island bridge to the gate. You will see a concrete wall with a fence on both sides. There is an invisible wall above the fence, preventing you from jumping over.
Drive to the left of the bridge and up the walkway. Position the ambulance so that its end lines up with the end of the black fence above the concrete wall. Exit the ambulance and jump up onto the roof.
Face the tree in front of you. Get a running start, then jump around the fence and onto the tree. Jump down from the tree to reach Star Fish Island.
Weapons in the Airport Terminal
Head to the airport and steal a car. Select a vehicle that's low enough to get into the airport's front gate, but high enough off the ground to reach the top of the metal detectors. The Sentinel is a fitting choice here. This vehicle will allow you to bypass the invisible wall above the guardrails.
Drive the car through the entrance of the terminal and park it up against the metal detector.
Exit the vehicle and jump on top of the car. Jump over the metal detector to gain access to the airport terminal. All of your weapons will be intact.
Getting a Boat
Steal an ambulance from the hospital. Drive it to the Pier 1 docks, which are behind a white building in the southwestern portion of the east island. You will see a large gate blocking access to the area.
Put the ambulance in reverse and back it up to the gate. Exit and jump onto the roof of the ambulance.
Walk up to the fence and jump through. Hold the "Up" button as you jump so that you will carry through the solid part of the fence. Once you're inside, take the boat docked on the pier. You now have access to all the other islands on the map, bypassing the invisible walls on the bridges.