Who owns an island?

I went on a bike safari through Southie yesterday. I was trying to ride out to the Harbor Islands that are connected to the mainland by a causeway. I had researched this via satellite imagery. I made it through the gritty industrial whatever, and as soon as I was about to hit the bridge, I got derailed by a rent-a-cop. Turns out the Harbor Islands are all private property. Who has enough money to own an island in Boston Harbor, and then still has enough left over to get a bridge built to it?