Is your above ground pool out of level? Did one side settle over the years? Was it a bad installation to begin with? Above ground pools do not have to be perfectly level to be safe. Pools look better when they are level but it is not always a safety hazard if they are a little off.
Aside from a pool visually not looking just right there is a way to tell exactly how far out of level your pool is. Knowing this will help you determine whether or not you want to fix it. With the pool full of water use a tape measure and take a measurement from the top rail to the water level. Do this in several different locations and compare measurements.
You know the water is level, it has to be. The pool, on the other hand, may not be. If your distance from the rail to the water in one spot is 6" and across the pool it measures 4" you know you have a problem, you have a pool out of level.
If you do have a pool out of level, and want to fix it, here is what I would do. Drain the pool down to just a few inches. Remove about five top rails in the low area and with one person inside the pool, pull the liner back away from the cove area. With the liner held in this position, the person on the outside can insert a round point shovel under the rails and foot plates, then slowly raise the pool. They can add dirt or a block under the footplate where it looks level. Make sure also that the bottom rail is supported. With that done the person on the inside reaches over to the inside of the pool liner and makes sure your cove is in place and add more if needed. Then reset the liner and put the rails back on.
This can also be done to a high spot by removing soil from under the bottom rails and footplates. It is easier to raise a pool than it is to lower one, but either way is fine. Keep in mind this fix is for pools a little off level, not a lot off level.
The two biggest problems you may have doing this are not replacing the cove, or draining too much water out of the pool, causing the liner to shrink. If this is a new installation the liner shrinking would not be a problem, it becomes a problem in the second or third year.
The next option is simply taking the pool down and starting over. Use a laser level on all of the footplates and get the pool level. That could even mean removing a sand base and leveling the ground. Liners do not fit well into a pool where the base is going downhill, even if the the rails and footplates have been raised to the correct level.How much is too far out of level? There are so many factors involved it is impossible to say for sure what is safe and what is not. A one inch drop at one post could do terrible things to a pool where a gradual three inch drop from one side to the other might be just fine. It also depends on the amount of dirt backfilled around the outside of the pool. A pool one foot in the ground could easily be six inches out of level and be just fine.
If you do have a pool sinking in one area, here are a couple of things to keep an eye on. Examine the wall closely and look for any signs of buckling. Also check the uprights and top rails. The uprights should not be pushed out of shape by the side wall. The top rails should be firmly attached with all the screws in their proper locations. Take a water level check, as mentioned above, on a regular basis. If the pool continues to become out of level it is probably time to consider fixing it.
For more detailed information about leveling the pool see my page Leveling an Above Ground Pool