when I did my epoxy/sand pieces, they wouldn't have a problem floating, but then the water only came up about ~1/2 way on them. The sand did add quite a bit of weight. If you want it to sink, you could hollow out some of the excess styrofoam after the expoxy resin has cured.
For the turtle island, I'm not sure if floating would be appropriate for a rock :P also, it may make it harder for the turtles to crawl on if it's not anchored. If it doesn't float, you could always put some supports in. or use a big piece of exposed driftwood.
I don't imagine it would be very easy to coat something with silicone, but I think it's a pretty common technique with the dendroboard folks.