The Florida Keys
Key Largo Water Temperature
Reef, flats, and Gulf Stream at the top of the Keys.
That is right about average for this time of year, and cooling over the last two weeks. The September average here is 87°F.
Updated Monday, September 21 — sea-surface temperature modeled for Key Largo by Open-Meteo.
For swimming
Very warm
Bath-warm. Pleasant, though it stops being much relief from the heat of the day.
For fishing
Hot — fish the edges of the day
Early and late is the play. Through the middle of the day fish drop into deeper channels and passes where there's moving water and more oxygen.
The last two weeks
Daily average sea-surface temperature, °F.
Key Largo 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 | 72°F | 67–77°F | Cool but comfortable |
| February | 75°F | 68–80°F | Warm |
| March | 77°F | 73–82°F | Warm |
| April | 79°F | 76–84°F | Warm |
| May | 84°F | 79–89°F | Very warm |
| June | 86°F | 83–89°F | Very warm |
| July | 88°F | 85–92°F | Bath water |
| August | 88°F | 85–92°F | Bath water |
| Septembernow | 87°F | 85–90°F | Very warm |
| October | 83°F | 79–87°F | Very warm |
| November | 78°F | 73–81°F | Warm |
| December | 75°F | 69–79°F | Warm |
Warmest on average in July at 88°F; coolest in January at 72°F.
The water at Key Largo
Key Largo is where the Keys begin and the fishing splits three ways: bonefish and permit on the shallow flats, snapper and grouper on the coral reef, and sailfish, mahi, and tuna in the Gulf Stream a few miles out. Tarpon prowl the channels and bridges, and the backcountry of Florida Bay opens up on the Gulf side. Few places offer this much variety in one run.
Florida Bay & 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 Largo; 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 Largo right now?
- The water at Key Largo is currently about 87°F (31°C). The September average here is 87°F.
- Is the water warm enough to swim at Key Largo in September?
- September averages 87°F at Key Largo, 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 Largo?
- July is the warmest month on average at 88°F, and January the coolest at 72°F. 11 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.