The Florida Keys
Key West Water Temperature
Flats, harbor tarpon, and blue water at the end of the road.
That is right about average for this time of year, and holding steady over the last two weeks. The September average here is 87°F.
Updated Sunday, September 20 — sea-surface temperature modeled for Key West by Open-Meteo.
For swimming
Bath water
Warmer than a heated pool. No cooling effect at all, and warm water like this holds less oxygen for the fish.
For fishing
Too hot for a midday bite
Water this warm carries noticeably less oxygen. Dawn is the bite and it's often over by mid-morning; the deeper passes and cuts hold the most life.
The last two weeks
Daily average sea-surface temperature, °F.
Key West water temperature by month
Averaged from 3 years of daily sea-surface temperature (2023–2025). The range column is the coolest and warmest daily average recorded in that month over the period — a guide to how much any given year can differ, not a record high and low.
| Month | Average | Range | Swimming |
|---|---|---|---|
| January | 75°F | 68–79°F | Warm |
| February | 76°F | 69–79°F | Warm |
| March | 77°F | 72–80°F | Warm |
| April | 79°F | 75–82°F | Warm |
| May | 83°F | 79–87°F | Very warm |
| June | 86°F | 83–88°F | Very warm |
| July | 88°F | 85–90°F | Bath water |
| August | 88°F | 85–90°F | Bath water |
| Septembernow | 87°F | 85–90°F | Very warm |
| October | 84°F | 81–86°F | Very warm |
| November | 81°F | 76–83°F | Warm |
| December | 77°F | 70–81°F | Warm |
Warmest on average in July at 88°F; coolest in January at 75°F.
The water at Key West
Key West, at the end of the island chain, offers everything: tarpon in the harbor and channels, permit and bonefish on the surrounding flats, yellowtail snapper and grouper on the reef, and sailfish, tuna, and mahi in the deep water a short run out. The Marquesas and backcountry to the west add legendary flats. It's arguably the most complete saltwater fishery in the country.
the harbor & the reef
What moves the water temperature here
The Gulf shelf off this coast is shallow, which is why the water swings so much more between summer and winter than open ocean at the same latitude does. Shallow water heats fast and gives that heat up fast: a single strong winter cold front can pull the inshore bays several degrees down in a couple of days, while the beaches a mile offshore barely move. Wind direction matters too — a sustained offshore breeze can pull cooler water up from below and drop the beach temperature noticeably in a morning.
Inshore bays, canals and flats run warmer than the beach through the summer and colder through the winter, for the same reason. The reading above is for the open water at Key West; expect the back bays to be a few degrees either side of it depending on the season.
Common questions
- What is the water temperature at Key West right now?
- The water at Key West is currently about 88°F (31°C). The September average here is 87°F.
- Is the water warm enough to swim at Key West in September?
- September averages 87°F at Key West, which most people would call very warm. Bath-warm. Pleasant, though it stops being much relief from the heat of the day.
- When is the water warmest at Key West?
- July is the warmest month on average at 88°F, and January the coolest at 75°F. 12 months of the year average 75°F or above.
- Why does the water temperature change so quickly here?
- The Gulf shelf off this coast is shallow, so it heats and cools far faster than deep open ocean. A strong winter cold front can pull the inshore water down several degrees in a couple of days, and a sustained offshore wind can drop beach temperatures in a single morning by pulling cooler water up from below.