Line a large, rimmed baking sheet with parchment paper, and preheat the oven to 275 degrees F.
In a large bowl, whisk egg white with water until frothy. (I do this with a fork or small whisk.) Add the pumpkin seeds and gently toss to coat. In a small bowl, combine the sugar, cinnamon, and salt. Pour the cinnamon-sugar mixture over the seeds and mix thoroughly.
Distribute the seeds in a single layer over the parchment-lined baking sheet (_without_ scraping the excess egg-sugar mixture from the bowl), and place in the oven. Bake, stirring every 15 minutes, until the seeds are dry and crisp. In my oven, this usually takes about 1 hour and 15 minutes. After the first 15 minutes, the mixture will look pretty soup and will be rather sticky. Tip: Scrape all the seeds towards the center of the baking sheet, being careful not to tear the parchment paper, and then redistribute the seeds over the baking sheet. As you bake and stir, the seeds will slowly dry out. Towards the end of the baking time, I make an effort to break up the big clusters and peel off the seeds that are sticking to the parchment paper. (You might find it helpful to let the baking sheet sit for a few minutes to cool just enough to use your hands to peel the clumps off the paper and then return it to the oven. Everyone likes the little clusters of seeds, so don’t worry about making them all separate.)
Store in an air-tight container at room temperature.