Horses have very poor depth perception, so what seems like a water jump to you may seem like an ocean to them. Try to desensitize your horse to water from the ground. If your horse is afraid, you can show him or her that an older horse goes through the water without being afraid, and lead them through together.
If he's afraid of it maybe show it to him from the ground and let him sniff it and have a good look at it! You could try getting him to walk over blue plastic or tarp first? Then try an empty water filler?
Then try it mounted!
Make sure your horse is straight, pushing into your hands from the leg, and sit back to be sure that you are not aged if the motion of your horse and can use your seat as a driving aide. Look up George Morris clinic Liverpool on YouTube and you should get some good results and ideas moving forward