If you want to reach the other islands before they are unlocked, then the video above shows you a couple of ways of doing this.
To reach Starfish island is easy as long as you can line up your jumps correctly. Take a motorbike, line it up on the left-hand side of the closed gate and then jump onto the wall and down onto the tree and you’ve made it inside the grounds of the Vercetti mansion.
Reaching the third and main island is a little harder and takes some trial and error. You need to park a vehicle in between the water and the grass on the left-hand side of the bridge from Prawn Island. Watch the video to see exactly how and where.
Then you need to get some height by jumping on the bonnet and you’ll be able to jump through the left-hand side of the bridge. Here’s where the trial and error comes in. Keep trying to walk as far forward as you can in the middle of the bridge. There’s an unseen barrier which prevents you from continuing, but if you jump in the middle of the bridge you’ll see your head emerge through the tarmac of the bridge and another jump will see you pop out through the road surface and onto the other side. From here you can just walk across the bridge and the island is unlocked.
Be careful though with what you get up to on the new islands as if you are wasted or get busted, then you’ll re-spawn back at the hospital or the police station on the first island and will have to do it all over again.