Whisk together water, kosher salt and sugar in a medium bowl (preferably with a pour spout). Heat in the microwave for 30 seconds on high. Whisk again to dissolve salt and sugar. If needed, repeat heating and whisking until salt and sugar are completely dissolved. Let brine cool completely before using.
To brine fish, cover fillets with brine, cover and let sit in the refrigerator for at least 30 minutes or up to overnight. The fish should be completely submerged in the brine, so scale up the recipe as needed. For example, I used 3 cups water, 3 tablespoons of salt and 3 teaspoons of sugar to make enough brine for the 1 1/2 pounds of salmon above.
Once you are ready to cook the fish, take the fillets from the brine, rinse under cold water and dry thoroughly.