You might tie a rope to something and throw the other end in your pool. Its very likely he will end up in there (if he hasnt left the yard) on a warm day and find it hard to get out again.
Try branches overhanging or near the pool if any, prefrerably any that could used to bask. Waterdragons tend to bask on sunny branches overhanging water, so they can dive in if they spot a predator.