Do a Google search for Fiberglass Osmotic Blisters. Some fiberglass boats that sit in the water for long periods get the same blisters. The way pools are manufactured is nearly identical to boat hulls.
I recently bought a 16' x 32' rectangular cover for my 16' x 31' pool. I had to trim about 1 foot off of the width and 3' off of the length. It seems that some solar blanket suppliers send over-sized covers.
This my 1st post, and I bought a house with a 30 year old in-ground pool in May of this year ('10). The advice from friends with pools & my agent was to just take a sample to the local pool store once a week and you'll be set. Well, if you're not from Florida there is a pool chain called...