Yep, the marines are the only way to take those islands. So I like to keep this in mind, so if I see a suitable place I'll build a city there, because I know I don't have to worry at all about defending it until marines come along (maybe a ship or two to defend against bombardments that could destroy the improvements). Two square islands are easy to defend also, just keep a worker or unit on the square that your city isn't built on. Then there is no way to invade the island without marines.