Use a lot of approach and retreat. Ask him forward and the second he hesitates stop and let him settle. It's not about the puddle. It's about his confidence in you. So, after he settles ask him forward again, and if he doesn't go simply ask him to back up, settle again, wait until he's relaxed then re-approach the puddle. The worst thing you can do is push him toward it, that will destroy his confidence. What I would do first, though, is find a puddle (or make one!) and work on this from the ground before working on it under saddle.