How odd. Can you enlist another family member to help in this? Have him on a leash, and right next to you. Let the other person enter the pool. Then you approach the pool, but as soon as he shows any aggression or snappiness, tug on the leash and tell him sit. This just makes him focus on you. Keep at it, and whenever he shows any reaction - refocus on you. Stay calm, don't yell, walk to the pool, around it, and away. Do this every day, and he'll get the idea that pool safety is NOT his responsibility, you are in charge there.

He's obviously thinking he's the pool boss - doesn't understand why you guys'd go in there voluntarily, probably!

Try that, see how it goes.