What’s the weather like in Key West? Beautiful! In Summer, the temperature is an average 89 degrees during the day to 80 at night. It can be humid with a slight breeze. The ocean temperature is an average of 85 degrees. These temperatures make for ideal water based activities where the water is crystal clear, warm and calm. In winter, the temperature is an average 75 degrees during the day to 68 at night. The ocean temperature is on average 74 degrees. These ideal temperatures attract many visitors in the winter months to enjoy a variety of activities on land and sea.