Why is my dog “attacking” other dogs?

My 3 yo labrador, Penny, is fine with about half the dogs she meets. The other half however, she “attacks”. She has never hurt another dog just growling and barking right next to their face. It looks scary to anyone watching. There is no difference between dogs she attacks and dogs she doesn’t that I have noticed. She has attacked our 6 year old collie over food many times, and yesterday she randomly attacked her while we were walking them and later while they were just sitting next to each other. She also recently “attacked” a puppy who was trying to play with her. We normally squirt her with water, say no, and separate the dogs. Is there a better way to separate the dogs? Is there a better way to teach her not to fight other dogs? (Also, she is completely fine with all humans)