I love the look of a pond with rock lining the inside, but when I built my pond I was on a tight budget so I skipped the rock. I didn't buy any rock - I scavenged them all at building sites and along the road. As said before, once the algae coats the sides of the liner, it doesn't look too bad. I also use a pond-vac to clean the bottom of the pond in the spring and fall, so having no rock on the bottom makes that easier.
I have always loved your pond @cas Though you used many of the materials that can make the ponds edg look like a string of pearls you did a greta job breaking up the edges with your plants, each plant has it's own shape and color and compliments the next just goes to show it doesn't need to be 2,000 pound boulders " (only some of us have rocks on the brain)"