Preheat oven 400 degrees Fahrenheit.

Bake the potatoes for 1 hour or until they are tender when pierced with a fork. Remove from oven and cool to room temperature. Slice potatoes in half lengthwise. Using a spoon, scoop the pulp from each half into a large bowl. Add the garlic and lemon to the potato pulp.
Using an electric mixer, beat the mixture until almost smooth. Mixing on low speed, slowly add the olive oil and vinegar, alternating between the two. Add the water and mix by hand until smooth. Add the parsley and season with salt and pepper; mix well. Serve at room temperature.