A concrete or fiberglass pool can float if the water level in the pool is significantly below the ground water level. The force pushing up on the pool by the water is equal to the weight of water displaced by the pool. Therefore, if the pool, and any water in it, weighs less than the weight of ground-water displaced, then the pool will float.
A simplified example: A 20 x 40 concrete pool with a uniform depth of 8 feet and 12-inch thick walls and floor would have a total concrete volume of 1916 cubic feet. Using a density of 150 lb./ft^3 for the concrete, the total weight of the concrete would be 287,400 pounds.
Using a density for water of 62.4 lb/cu. ft, the pool would have to displace 4,606 cubic feet of water to be neutrally buoyant.
Therefore, the ground water would have to be 5 feet up from the bottom of the floor of the empty pool to be neutrally buoyant. That's 4 feet down from the top edge of the wall.
If the pool were a uniform 5 foot depth, then the ground water would have to be 4 feet up from the bottom of the floor of the empty pool to be neutrally buoyant. That's 2 foot down from the top edge of the wall.
Note: Wall and floor thickness varies. 12 inches is thicker than most pools. A thinner wall will weigh less, and float more easily.
You have to use the thickness of the wall in question to get a correct answer. Also, most pools vary in depth, which makes it more complicated to do the calculations. In any case, ground water needs to be taken seriously when draining any type of pool.