Water temperature in Marathon Key (FL) in December

Is it warm enough to swim in Marathon Key in December? Average water temperature in Marathon Key in December is 76.6°F and therefore suitable for comfortable swimming. The warmest sea in Marathon Key in December is 80.8°F, and the coldest is 72.5°F. To find out the sea temperature today and in the coming days, go to Current sea temperature in Marathon Key

Changes water temperature in Marathon Key in December 2025, 2024

To get an accurate forecast for the water temperature in Marathon Key for any chosen month, compare two years within a 10 year range using the chart below. In addition to the sea water temperature in Marathon Key, the selected years in the table also show the values of the maximum and minimum temperatures on each of the days of December.

Average water temperature in Marathon Key in December over a decade

Chart comparing the height of waves in Marathon Key in December over the past years. The red line indicates the height of the waves when swimming in the sea is prohibited. To view the current sea conditions, click on the graph.

You can also view information about water temperature for other months by clicking on the month button below

The nearest locations with the average water temperature in December. In addition, the maximum and minimum water temperatures for that month, recorded over the last decade, are shown

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Our report for Marathon Key is compiled using satellite data together with in-situ observations to get the most reliable daily data of sea surface temperatures, surf forecasts, current temperatures and weather forecasts.