I would deal with it the same way as getting them to cross anything else. Take my 2 year old today for example. I took him to the river to get his feet wet. First, I got off and led him in, rubbed him all over, he played in the water. I got on him, rode him out, made a loop and went to the edge of the water and asked him to go forward. He took a small step, which I rewarded, then he would not budge and wanted to back up. So we backed up until he decided that was no fun. Went back to the water, asked for forward again, got a foot in the water then he wanted to back up again, so we did. Third time at the edge, I asked him to go forward and he kind of lept in. We stood knee deep for a bit, he pawed the water, I pet him all over and told him what a good boy he was, then we got out. I rode him in a loop again, then asked him to get in the water, he went right in. This is how I go about everthing a horse doesn't want to cross.