I don't know about the PS2 version, but on the 360 version, you can see the island from the shore. Also, it's visible on the map, and you can clearly see the road surface on it, something none of the other isolated islands has.
On the 360, there's a marker at the top of one of the mountains near the shore. Not easy to get to. It's the logo of Eden Studios, who made the 360/PC versions. Drive through it at a certain speed (50-60mph, I think, or faster) and it will teleport you to the island. There's another icon/marker on the island to teleport back. But, since Eden didn't make the PS2 version, there's a strong chance this isn't there, or it's been altered or moved.
There really isn't much there, though. I've only been there once and never bothered going back. I don't know if it counts toward total percentage of discovered roads or not.