In a non-reactive saucepan, bring water, vinegar, salt, and sugar to a boil. Simmer until sugar and salt are dissolved. remove from heat and add peppercorns and garlic.
Pack radishes into a jar and pour brine over to submerge. Cover and let cool to room temperature before refrigerating.
Made with assorted radishes and kosher salt instead of sea salt. About 5 medium radishes yielded a packed quart jar full, and I had to waste the extra brine (should have weighted the radishes). I crushed the garlic cloves before adding to the jar.