In a medium bowl, whisk together the water and sugar to dissolve the sugar, then stir in the sherry vinegar and soy sauce.
Bring a large pot of water to a boil. Carefully put the eggs into the boiling water and cook for exactly 6 minutes and 50 seconds, stirring slowly for the first 1 1/2 minutes to distribute the heat evenly. Meanwhile, fill a large bowl with cold water and ice. When the eggs are done, transfer them to the ice bath.
Once the eggs are cool , peel them (in the water—this will help them keep a perfect exterior). Transfer the eggs to the soy sauce mixture and marinate in the fridge for at least 2, and up to 6, hours, making sure they are completely submerged. If necessary, top the eggs with a small plate to ensure submersion.
Remove the eggs from the sweet and salty solution. You can save the soy sauce mix for another round of eggs, if you wish. The eggs will keep, refrigerated in a tightly sealed container, for up to a month.
To serve, cut the eggs in half lengthwise and season with salt and pepper.