December to February is high season, when Punta del Este becomes the most fashionable resort in South America. The beaches are packed, prices are high (sometimes extremely so), and the energy is genuine and hard to find elsewhere on the continent. New Year's Eve draws particularly large crowds.
March and November are the shoulder months — still warm enough for the beach, far cheaper, and significantly less crowded. These are arguably the best months to visit if you want to see the place without fighting for space.
April to October is the off-season. Many restaurants and hotels close or reduce hours. The beaches are empty, the air is cool (temperatures drop to around 10°C in July), and the resort has the quiet, slightly melancholic atmosphere of a summer town in winter. The landscape remains beautiful and the seafood is still excellent, but come prepared for a subdued experience.
Water temperatures peak in February at around 22–24°C on the Brava side and a couple of degrees cooler on the Mansa side.