As Ray Hunt would say "Make the wrong thing difficult and the right thing easy."  You make the  water the best place to be.  As you are approaching the water if your horse begins to hesitate get them busy moving their feet. Make a circle, turn right, turn left and get them moving and then point them to the water and ask them to move forward petting them and encouraging forward.  If they begin to hesitate then repeat the process.  The important thing to remember is the timing sending them to the water and the feel of when they are hesitating. You have to have timing and feel of the horse so that you can feel them make the change towards the water and reward them.  The process can take some time and patience.  But if you are consistent, patient and have good timing and feel you can get the job done.