I am going to make the assumption that you are going to use a boat to move the sandbags or whatever materials, since you are talking 90" off shore.
So how about, not exactly an island, but a series of plastic barrels filled with the dirt that you were intending to use. Place the barrels as close or far apart as you wish. You could drill holes in them to let them sink or cut out the entire bottom and fill them in place or you could raft them up at shore line and partially fill them, then float them to your spot and fill the remainder of the way, sinking them in place.
And as opposed to top soil, use clay for the planting layer. It holds together better in water and keeps the plants from floating away.
It could be a project that you could add to over the weeks/months/years.
There were suggestions for locating the barrels for free in other posts.
