The equation for depth involves x, the distance from the shallow end. The pool is 25 meters long. Given this information, what do you think the limits should be?
Correct. Generally, whenever you need to integrate something of a fixed length l, your interval should be from 0 to l. This includes when finding area, volume, time, etc, which if you haven't done yet you will. You'll always get your interval from the problem; sometimes you may have to solve for your limits of integration.