Lung them in a circle close to the water or create your own puddle. Then make your circle bigger so they get closer and closer until they are trotting or cantering through the water.
Just be patient and you will get there also getting another horse to be a lead is a good help and you could always try leading you horse in on foot that has helped with all my horses who dislike water!
Hope this helps
I would recommend walking your horse through the water a few times to let them know it's safe.