Preheat oven to 400°F (200°C). Line two baking sheets with parchment paper and set aside.

In a medium saucepan, combine water, butter, granulated sugar, and salt. Bring to a rolling boil over medium-high heat, stirring occasionally until butter melts completely.

Remove pan from heat and add all the flour at once. Stir vigorously with a wooden spoon until the mixture forms a smooth ball and pulls away from the sides of the pan, about 1 minute.

Transfer dough to a mixing bowl and let cool for 5 minutes. Add eggs one at a time, beating well after each addition until the dough is smooth and glossy. The mixture should be thick but pipeable.

Transfer dough to a piping bag fitted with a large round tip. Pipe 4-inch logs onto prepared baking sheets, spacing them 2 inches apart. Use a wet finger to smooth any peaks.

Bake for 20 minutes at 400°F, then reduce heat to 350°F (175°C) and bake 15-20 minutes more until deep golden brown and hollow-sounding. Do not open the oven during the first 20 minutes.

Remove from oven and immediately poke a small hole in each end of every eclair with a skewer to release steam. Cool completely on a wire rack.

In a large bowl, beat heavy cream, 1/4 cup powdered sugar, and 1 teaspoon vanilla until stiff peaks form. Gently fold in the diced peaches. Transfer to a piping bag fitted with a small round tip.

Insert the piping tip into one end hole of each cooled eclair and pipe filling until cream just begins to emerge from the opposite end. Wipe any excess and place filled eclairs on a serving platter.

For the glaze, whisk together 1 1/2 cups powdered sugar, milk, and 1/2 teaspoon vanilla until smooth. Dip the top of each eclair into the glaze, letting excess drip off, then return to platter. Refrigerate for 15 minutes to set the glaze before serving.