Generally, it is never too cold to swim in Key West, as the water remains relatively warm year-round due to the Gulf Stream. Even in the depths of winter (January and February), water temperatures typically hover between 70°F and 74°F (21°C–23°C). While some locals might find this a bit "brisk," most visitors from northern climates find it perfectly comfortable for a quick dip or snorkeling. From March through October, the water warms significantly, peaking in the high 80s (30°C+) during the summer months. If you are particularly sensitive to cold, you might prefer the "bathwater" temperatures of June through September, but for the average traveler, the ocean in Key West is welcoming 365 days a year. However, be aware that occasional "cold snaps" can temporarily drop surface temperatures for a few days, so checking a local forecast before heading to Smathers Beach is always a good idea.