Check your local libarary for a book on building ponds. It explains
how to dig the hole with two layers so that you can have plants that
thrive in shallow and deep water.
You right about the rock bottom, I think that's best. You will still
need a liner, but it's thick plastic instead of the fiberglass. You
buy it in a big sheet and cut it to size. You'll need plan your pond
according to the amount of liner you can afford.
If you just use rock the water will eventually seep out. So I'm afraid
you'll still need the plastic liner.
You can price it out at Home Depot, Lowes or a Garden Specialty store.
My pond is made this way, and so is the waterfall. You get a much more
custom look to it. I would take a photo of the pond but right now it's
frozen. hehe. it's 18 degrees outside.