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 minute. 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, (5-7 minutes) peel them in the water. See notes above.
Transfer the eggs to the soy sauce mixture and marinate in the fridge for at least 2 hours and up to 4 hours, making sure they are completely submerged or occasionally rotate eggs.
Remove the eggs from the sweet and salty marinade. You can save the soy sauce mix for another round of eggs, if you wish. Refrigerate eggs in a tightly sealed container.