The way to get to the following island earlier,(Portland to Staunton, Staunton to Shoreside), you must have the Flying Cars cheat on and a fast car(or a Yardie Lobo). You must drive fast towards the broken bridges and angle your hood of the car up to make a good landing to the other side. With a Yardie Lobo, you can use the hydrolics to get a better jump and land easier.