you can do volume=length times width times height for a perfectly rectangular pool
you can get lots of tennis balls and fill the pool with them and estimate the volume, that's how they measure the volume of wierd spaces in cars.
you can fill the pool with water and measure how many gallons in the pool then convert that to square feet
you can put a scanner (special type) and make a 3d rendering of the pool then measure the volume with the tools in the 3d program
