As you come down to the roundabout heading to the sea, the funfair is on your left. If you go past teh enrance to the fair and go diagonally right, you hit teh cordoned off swim area after about two mins. You can actually swim pretty much anywhere along that section but don't go in the water at either the far left or far right (especially) of the island. The current is murderous. The cordoned off bit is fine.
If you do make it to the island, at the funfair roundabout facing the sea, go left for a mile to Eastoke Corner and have a Pepperellis ice cream. It's not a recommendation I make lightly!