I've tried a few different 'scapes with a "land" portion. This is a pic of my pool when I attempted a mangrove island/beach.
The sand would wick water and never dried out, even though in places it was at least 12" above the water's surface. After about a month, it started to develop a "wet laundry" smell. Wasn't too unbearable, but definitely noticeable if you got close enough. I ultimately decided to "sink" the beach, but still do have a couple pieces of rock that peak above the surface without any issues. Anyway... just something to keep in mind if you should decide to try some sort of island