Setting up an 18 mile connection to an island should be a little easier than an 18 mile land connection. That is if the majority of the space in between is ocean. You will not have to deal with buildings or trees in the path. You should have not trouble with line of sight with this setup. It will take a little while for you to get the antennas adjusted perfectly. As far as stability, there will be less chance of physical interference and probably not a lot of radio interference unless there is a boat big enough that comes in the path.
