Somewhere there is/was a thread with pictures of the meeting held at TexReefers house where Great Stuff rock was demonstraded being made. I just couldn't find it when I searched.

Yes, it does float. You will have to weigh it down with something to keep it in place.

When the stuff is still wet, sprinkle crushed coral or something similar on it for texture.

For larger pieces, you have to layer the stuff on or it will collapse. Spray some, let it start to cure a bit, then spray on some more. Use eggcrate for structure.