Let's take a spin through Free Stuff:

jbezorg has a Rock and Spire Prop.
MikeJ has a couple of rock props.
86Corvette has an island-like item called Dark Rock, for Bryce.

Maybe one of those is what you're looking for.

If you need other underwater props, be sure to visit Mitch's 3D Seite. It's loaded with diving gear, aquatic plants, seashells, and a fantastic minisub.