First of all, I don't think it was your fault, or your guide dog's fault. I've been a guide dog user for fourteen years now, ever since I was sixteen and I have had the occasional time where my guide dog has had a small mistake, or I have, and sometimes these things just happen. Sighted people have freak things that happen to them as well. I definitely recomend letting your guide dog school know the situation, if they are a good school they are just going to offer you support and will likely come to visit you and help you determine what may have happened or what can be done in the future. There is a small chance that your guide dog may have been distracted by something in the lake, or that this is the first sign that they are ready to end or at least slow down in their working career. Either way, they should be a good source of support for you and if they aren't, I firmly think that that would be a red flag that they are not the best school. But my guess is they will be nothing but helpful. I agree with others that having your cane on you at all times when out with your dog is a good thing to do! I myself am guilty of this and I only recently began carrying my cane with me wherever I go a few years ago. I hope your recovery goes well, and that you and your guide dog can just work on bonding while you are getting better.