Dave, If you print a large ref chart it may be as simple as taping it to the side of the pool. You might try just leaving enough extra material so you can tape it above the water line. Depends on the lens and distance your trying to test.
John, I can't decide if your last comment was the scurilous hack talking or the intrepid reviewer. Just curious, what percentage of the images that you get sent to you would you consider good enough to print?