Bath time is no big deal, I train young and old horses all the time to like water, all you do is back them up when they move or spook from the water, spooking is a bad thing, and horses can be trained not to spook, they will still spook here and there but still work them and tell them that spooking is a bad thing. If I can train horses to like a tarp and be able to stand in the saddle and let it blow it over my head and not have them spook and then you can train them to not spook from the water.
If you make it a big deal and then your horse will every time.