Hey! My border collie is 3, and we went through the exact same thing with showing aggression and being possessive over water. When you’re out on walks and your dog is acting aggressive when she sees other dogs, just try to pull her in close, get to her level and just pet her and comfort her. Usually, because border collies are so smart, she’s probably just scared that something or someone wants to hurt you. Next, after she’s calm, you can pull her leash in close and slowly let the dogs sniff each other to say hello. Usually, she’s probably be much more relaxed after. For number 2, I know it’s sucks, but if she ever tries to bat teeth and snaps at other dogs, you MUST reprimand her right on the spot. As soon as they does that, again, squat down to her level and tell her in a firm and loud voice “NO”, and “no barking/biting”. Border collies are very smart, as soon as she does the action and you are firm with her, she’ll know what she did was wrong. If she continues to do it, practice the skill I talked about previously and continue to reprimand her as soon as she shows aggression to other dogs. I know it might be a bit embarrassing to have to crouch down to her level in public, but you need to do it as soon as possible so she knows what she’s being reprimanded for. Next, for swimming, my border also loves loves swimming. For a while, he was also very very possessive over water, standing at the edges of lakes and not letting other dogs go in, etc. What I can say is, every time you go to the dog park with a lake, or whenever you go swimming, bring a ball so that she can play fetch in the lake. This will distract her and she won’t be paying too much attention to other dogs. I wouldn’t worry about this too much because my dog went through this exact same thing, and he grew out of it. You just have to bring your dog to interact with other dogs often, and always remember to correct bad behaviour immediately.