Yes, horses can swim very well, I swim with mine in the summer time. One thing you'll need to do is take your time getting her used to being out in the deep water. Some horses panic the first time their feet lose touch with the ground. I would stay square on her back for the first few times until you know she's going to be calm about the water. After you know she's okay with it, then you'll need to start sliding off her side and swimming beside her (or just hanging onto her mane and letting her pull you). While horses can swim with the rider straight up on their back, it tires them out quickly. With you swimming beside, she could likely make it all the way across a decent sized pond.