The Florida Keys
Marathon Water Temperature
Seven Mile Bridge tarpon and Middle Keys reef.
Sea-surface temperature
Very warmHolding steady · last two weeks
That is about 2°F warmer than average for this time of year, and holding steady over the last two weeks. The November average here is 80°F.
Updated Saturday, November 7 — sea-surface temperature modeled for Marathon 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
| Date | Average |
|---|---|
| Oct 25 | 82°F |
| Oct 26 | 82.2°F |
| Oct 27 | 81.9°F |
| Oct 28 | 81.5°F |
| Oct 29 | 82.4°F |
| Oct 30 | 82.8°F |
| Oct 31 | 83.1°F |
| Nov 1 | 82.8°F |
| Nov 2 | 82.6°F |
| Nov 3 | 82.4°F |
| Nov 4 | 81.7°F |
| Nov 5 | 81.9°F |
| Nov 6 | 82°F |
| Nov 7 | 82.2°F |
Marathon 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 | Range | Average |
|---|---|---|
JanJanuary Cool but comfortable | 67–79°F | 74°F |
FebFebruary Warm | 68–80°F | 75°F |
MarMarch Warm | 73–82°F | 77°F |
AprApril Warm | 77–83°F | 79°F |
MayMay Very warm | 79–88°F | 84°F |
JunJune Very warm | 83–90°F | 86°F |
JulJuly Bath water | 85–92°F | 88°F |
AugAugust Bath water | 84–91°F | 89°F |
SepSeptember Very warm | 85–90°F | 87°F |
OctOctober Very warm | 80–87°F | 84°F |
NovNovembernow Warm | 74–83°F | 80°F |
DecDecember Warm | 70–80°F | 76°F |
Warmest
89°FAugust
Coolest
74°FJanuary
Warmest on average in August at 89°F; coolest in January at 74°F.
The water at Marathon
Marathon in the Middle Keys is bracketed by bridges — the Seven Mile and the old Bahia Honda spans — that hold tarpon through the spring and summer. Permit and bonefish work the surrounding flats, yellowtail and mangrove snapper light up the reef, and mahi and tuna wait in the deep. It's a well-rounded base for reef, flats, and offshore in one place.
Vaca Key & 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 Marathon; 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 Marathon right now?
- The water at Marathon is currently about 82°F (28°C). The November average here is 80°F.
- Is the water warm enough to swim at Marathon in November?
- November averages 80°F at Marathon, which most people would call warm. About as good as it gets — warm enough to stay in for hours without getting chilled.
- When is the water warmest at Marathon?
- August is the warmest month on average at 89°F, and January the coolest at 74°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.