The BIG problem is not the dead grass, it's the compaction that results from the weight of the water...
We have a 12' wide by 3' deep pool for our kids who just turned 6. It's not too small where they think it's "for babies" but not too big where they feel unsafe (they can stand up in it and the water is only up to their chests). The pool holds 1700 gallons. Multiply that by 8.34lbs. per gallon and you're talking about 14,178 pounds (7.1 TONS) of water sitting in a 12 foot circle on top of your grass. Taking the math further, a 12' circle has an area of 113 sq. ft, so each square foot is seeing 125lbs. of weight pressing down on it. This compacts the living hell out of the soil so even if you rake the dead stuff up you have to do something about aerating the soil.