I agree with riosdad. I've done this with 5-7 horses and it always pays off.
One thing I'd suggest, if you don't mind, is taking the hose and really topping off that puddle in the gate so there are no areas he can slip through to avoid the puddle. It'll dry up.
A: Never stand on one side of a puddle directly in front of the horse and try to pull it across. Chances are very good that it will decide to jump the puddle and jump straight for you. NOT fun. Learned that one at about 13 years old. Haha.
So, lead the horse to the gate/puddle and don't even look at it or the horse, just keep walking forward. The horse will inevitably stop. Walk right into the puddle, and slosh around. Walk back out. Circle around, repeat. If it is really taking a long time and is a huge struggle, consider carrying a dressage whip or lunge whip in your off hand facing backwards. Tap the horse as you are walking forwards to encourage that forward movement.
I've resorted to standing in the water and sloshing it up on the horses feet. Eventually they are desensitized to it.
It really just takes persistence. The horse has to go in the paddock, and he will. Just don't get mad and angry and take it out on the horse.
We spent 3 hours out on the trail getting a horse to cross a 2ft wide creek. He eventually gave in, we walked through it 5 more times, and he didn't care at all anymore.
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