So try and show him that the water is not gonna hurt him and you go threw it first, then so he sees that you are not hurt, then if he doesn't go to that force him show him who's in charge... What I mean is take the reins and pull him slowly step by step.
First, I would start out by giving sugar to keep to your horse calm. Then, once he gets comfortable put water in a liver pool and put carrots in the middle of it. After he gets comfortable I would start leading him in the water slowly. Gradually, I would make the water higher as your horse’s confidence increases. Hope this helps answer your question!
With my baby’s I just led them up to it and let some take a look and reassure them also with my older ones I lunge them near it then slowly work my way closer towards the water to the point where they are happily going through it!
Hope this helps