Whether the sea in Mykonos feels "cold" depends heavily on the month of your visit. During the peak summer months of July, August, and September, the water temperature reaches a very pleasant 24°C to 25°C (75°F to 77°F), which most swimmers find ideal for long dips. However, in the late spring (May and June), the water is still "refreshing," usually hovering around 19°C to 22°C. If you visit in the "off-season" from January to April, the water is definitively cold, dropping as low as 15°C (59°F), which is generally considered too chilly for traditional swimming without a wetsuit. A unique factor in Mykonos is the "Meltemi" wind—strong north winds that blow in the summer; while they keep the air temperature comfortable, they can create choppy surface conditions and occasionally move surface water around, though they don't significantly drop the actual temperature. For the best "beach vacation" experience where the sea feels like a warm bath, late August and early September remain the golden windows for travelers.