How do I check the area of my 18 ft round pool to see how level it is?

How do i check the area of my 18 ft round pool to see how level it is?
A carpenter's level contains a varying number of bubble vials mounted in a wood, plastic, or metal frame. Use this tool to check that surfaces are level (horizontally aligned) or plumb (vertically aligned).I used this for my 18X48 Pool.

You can buy water level for checking large areas. The level runs about $30 and uses a 30" plastic tube of water connected to a small box. You establish the box at the elevation you want and then move the tube around. The water level will always match the box and set off a tone when it is level. A carpenter's level, even a mason's level, would be a stretch for 18"