Myself and 2 of my friends found a glitch on prawn island. You can walk inside of the brick walls surrounding the homes of the north part of the island.
You can walk further down the wall, or even drive a PCJ through it as my friend did here:
And lastly, if you get off of your PCJ, you will teleport to Little Havana, on the road between the Donut Shop and the PD. Weird, isn't it?