I have a few of those granite rocks but in a smaller tank. In my 90 gallon they just didn't fit right, they looked tiny. It would only really work out if you covered the entire bottom with them, but i don't see that working very well. I got lucky though and was able to swipe some large round river stones from the architectural design firm next door to my office.
I was going to go look around the Rickenbacker to see what I could find out there as far as driftwood goes. Im not very hopeful, but my dog likes the swim so I end up there often regardless.
What about the area around Old Cutler? has anyone tried that area out? they have all those mangroves if I remember correctly. I have no car so its tough to get out there.