Another plunger method I used last year (Computer room tile floor puller).

We had a ton of rain and the rain water ended up getting behind the liner. Here's what I did:

1. Bought a really cheap pump at Lowes that included a piece of garden hose, I put it behind the liner and I guess once you get the water just low enough...the force of the pool water all of a sudden pushes all the water back out in the rock area behind the pool.
2. Anyway, this left wrinkles in the shallow end so I brought one of the computer room floor tile pullers from work. (I don't know what they are called) I didn't drain any water and I was able to pull all of the wrinkles out of the bottom. It worked really well and I'm sure works similar to the plungers, but this worked so well because you can pull and push pretty hard before it releases from the liner.