Teaching Bodies of Water Safely
Many horses are afraid of large bodies of water - and for good reason. They can't see where their feet would land, how soon they would need to swim, and if there are any horse-eating-boogie-monsters under the water. I introduce horses to water from the ground. I know some people prefer to ride in, but I've found horses tend to be less nervous when they feel they're doing it with or after their leader.
When I bring a horse into water, I choose a safe entry point, typically one I've checked beforehand for any hazards under the water as well. Then, I enlist a helper. Two ropes are attached to the halter, one to the chin ring that I hold, and one to the side where the helper will hold. This is done for a couple of reasons, namely so the horse can see someone entering the water on either side, so he won't feel quite as apprehensive. It's also for safety reasons. Walking in knee-deep lake mud means you're not only moving slower, but you're barefoot and lack a lot of the clothing that would typically keep you protected from a nick or scrape. If a horse were to panic while entering the water, they would typically come towards the person holding them, sometimes resulting in disaster. Having two ropes of equal length keeps the horse from ending up on top of either person.
Then we walk the horse into the water together. It's important to make this a POSITIVE experience. No whipping, hauling, kicking, etc. if the horse doesn't want to go in, accept that you didn't spend enough time preparing him and come back another day. Once the horse is in deep enough, the helper releases their rope to the horse has more freedom of movement. While on land this would cause problems, it doesn't in the water. In fact, it actually helps. First, the rope doesn't get stepped on because most nylon lead ropes float. Even if they do get caught, the mud is deep enough that they just slide out with essentially no resistance. They also help in exiting the water. When getting out, my horse is typically a lot more balanced and a lot faster than I am. I have a helper wait by the entrance point, and guide the horse toward them.
Once the helper has grabbed the secondary loose rope, I release mine and they gather that up as well. This way, the horse is never loose in and around the water, and everyone is kept safe. Feel free to ask any questions below!