Peel the zucchini completely and cut it into uniform 1/4-inch slices. Measure 2 cups. If using an overgrown zucchini with a hard, spongy center, remove that center and slice the firm outer flesh.
Put the dill in a clean 16-ounce glass jar, or divide it between two 8-ounce jars. Pack the peeled zucchini slices over the dill without crushing or bending them.
Combine the filtered water, chosen vinegar, sea salt, and optional smashed garlic in a small stainless-steel saucepan. Stir over medium-high heat until no salt crystals remain.
Bring the brine to a full boil, then remove the pan from the heat. Let the brine cool for 5 to 10 minutes.
Pour the hot brine slowly over the packed zucchini until the slices are covered. Tap the jar gently to release trapped air. Add an optional glass weight while the jar cools.
Leave the jar uncovered until mostly cool. Add the lid and refrigerate for 24 to 48 hours before eating. Regular zucchini should taste tart through the center and remain crisp-tender; extra-large zucchini needs at least the full 48 hours.