Lunging at other dogs on walks

[Zoey]

My 5.5 month old GS pup is incredible on walks with no one else is around, when were in the field we play fetch in she will follow me and stop when I do, I could win a competition when were alone. When we walk around a public lake with many people walking there dogs as we approach them she will lunge when they go by, even when I have her sitting calmly before they approach as soon as the other dog is close or passes she lunges at them.
Now I know she just wants to play because if I let them meet she sniffs then wants to romp around and play with the other dog, the problem is most of the other people are not so into it, there dog seems interested not the snooty people. I am trying to socialize her more with the lake walks but I am finding myself avoiding it most of the time so not to deal with it. Should I walk her there more so she might start to get used to other dogs and stop lunging? what do you recommend to help with this lunging?
She does go to a dog day care to play with other dogs maybe once a week. Thanks

[enae]

Take a small squirt bottle filled with water with you on your walks. Watch the dog's head when you see another dog coming your way. As SOON as she notices the other dog you will see a change in her ears and other body language. Give her a squirt right on the nose with a verbal correction. It is much much easier to correct the "thought" before the action. When you try to give corrections when she is already in action, she thinks you are joining in.