I use rocks for lots more than ponds. I always use the same kind of rocks, called Arkansas Moss Rock. It's a fieldstone and can be gotten in flat fairly good sized pieces. It's hard to build a waterfall with little rocks. Since we do not grow rocks in Louisiana, we must buy them by the pound. They run about 35 cents a pound and when you need tons, you are talking big bucks.
Often after the pond is in, I will put in a path using more flat rocks. Occasionally building a terrace by placing them piled up against a raised bed to anchor the bed will be an effective accent in an otherwise flat landscape.