I had the very same issues with my horse when riding with a large group of horses on the trail. The more exposure she got to riding in large groups helped tremendously. Sometimes she and I travelled twice as much ground as most of the others, because of all the circling I made her do.
For getting your horse to go into water keep riding with someone who has a horse that will go in with no hesitation. Also when your approaching a water situation visualize your horse going in like he has done it many times successfully. If you get tense and anticipate his refusing to enter the water your horse will "feed off" your tension.
Happy Trail Riding..