Stir water, vinegar, sugar, and salt together in a saucepan over high heat. Bring to a boil; remove from heat and cool completely.
Combine cucumber spears, garlic cloves, dried peppers / crush red pepper flakes, yellow mustard seeds, and fresh dill in a large glass or plastic container.
Pour cooled vinegar mixture over cucumber mixture. Seal container with lid and refrigerate for at least 3 days.