Georgia Coast
St. Marys Water Temperature
Cumberland Sound, the St. Marys River, and the ferry to Georgia's wildest island.
That is about 7°F warmer than average for this time of year, and cooling over the last two weeks. The October average here is 75°F.
Updated Friday, October 2 — sea-surface temperature modeled for St. Marys by Open-Meteo.
Out in the open Gulf, NOAA buoy 41112 — 15 miles offshore — is reading 81°F. Deep water offshore runs cooler than the shallows in summer and warmer in winter, so it won't match the beach.
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.
St. Marys 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 | 57°F | 49–61°F | Cold |
| February | 59°F | 55–67°F | Cold |
| March | 64°F | 58–70°F | Bracing |
| April | 71°F | 66–76°F | Cool but comfortable |
| May | 78°F | 73–83°F | Warm |
| June | 82°F | 76–86°F | Very warm |
| July | 85°F | 83–89°F | Very warm |
| August | 85°F | 79–89°F | Very warm |
| September | 82°F | 79–85°F | Very warm |
| Octobernow | 75°F | 70–82°F | Warm |
| November | 70°F | 63–76°F | Cool but comfortable |
| December | 61°F | 57–66°F | Bracing |
Warmest on average in July at 85°F; coolest in January at 57°F.
The water at St. Marys
St. Marys is the last town on the Georgia coast, sitting on a blackwater river that forms the Florida line and drains into Cumberland Sound. The fishing is classic Georgia estuary work on enormous tides: redfish and trout along the spartina and the oyster rakes, flounder in the sandy cuts, and black drum and sheepshead tight to pilings and hard structure once the water cools. Plenty of people here are headed somewhere else, though — the ferry runs out to Cumberland Island, an undeveloped barrier island of live oak forest, wild horses, and miles of empty beach, and every hour of that day depends on the weather and the boat schedule. Summer thunderstorms build over this coast in a hurry.
the St. Marys River & Cumberland Sound
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 St. Marys; 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 St. Marys right now?
- The water at St. Marys is currently about 82°F (28°C). The October average here is 75°F.
- Is the water warm enough to swim at St. Marys in October?
- October averages 75°F at St. Marys, 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 St. Marys?
- July is the warmest month on average at 85°F, and January the coolest at 57°F. 6 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.