You say your pool has rocks arnd it but is it a kiddy pool or man made? Ours will be getting together soon, the snows just gone, but the nights haven't warmed up much yet. 17 degrees last night. Were working on the ducks house right now. Then it will be the wet section to figure out. Kiddy pool or what we're still up for suggestions! Reading the net,seems everyone says kiddy pools the easiest.
Since you only have a couple of ducks you may want to try something like this (which is what I'm doing):
http://www.homedepot.com/p/Beckett-35-Gal-Plastic-Patio-Pond-PP1035/202206168
At thirty-five bucks it's not too pricey. It's deeper than a kiddie pool and far more sturdy.
Last year I plumbed a kiddie pool with a drain and pipe to make emptying easy. Just didn't hold up that well. At least you're not out much if you go that route. The pond above I'll be raising above ground using dirt and retaining bricks. I'll also plumb it to drain the water into the garden. Same concept, just a little more permanent.