You’re standing on the edge of the dock at Sombrero Beach. The sun is doing its thing, that heavy, golden Florida Keys squint-fest, and the Atlantic looks like a sheet of turquoise glass. You want to jump in. But there is that nagging question: is it going to feel like a refreshing dip or a lukewarm bath? Or, if it’s January, is it going to knock the wind out of you?
Honestly, marathon fl water temp is one of those things people gloss over until they’re actually shivering in a 3mm shorty wetsuit or sweating in the middle of the ocean.
People think the Keys are "tropical" year-round. While that’s mostly true for the air, the water has a mind of its own. It reacts to the shallow shelf of the Florida Bay and the deep pull of the Gulf Stream. If you’re planning to fish, dive the Thunderbolt wreck, or just float with a margarita, the temperature under the surface is the only metric that actually matters.
The Reality of Winter: It’s Not Always Bath Water
Let’s kill the myth right now. From late December through February, the water in Marathon isn't always "warm."
If a north wind kicks up—what we locals call a "blue norther"—the shallow waters of the Florida Bay can drop into the 60s fast. On average, you’re looking at 71°F to 74°F during the winter months. For someone coming from Maine, that sounds like a dream. For a local? That’s "I’m not getting in without neoprene" weather.
January is statistically the coldest month. The NOAA station at Vaca Key (VCAF1) often clocks temperatures around 71°F or 72°F. If you’re snorkeling at Sombrero Reef during this time, the wind chill when you climb back on the boat is actually the part that hurts.
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- December: ~75°F (Still decent, but cooling)
- January: ~72°F (The "brisk" peak)
- February: ~72°F (Holding steady)
If you're diving, you’ll want a 5mm full suit if you chill easily. Snorkelers can usually get away with a 3mm shorty, but don't expect to stay in for two hours without feeling the drain on your core temp.
Spring and the "Sweet Spot"
March is the transition. It’s the month where the water temperature starts its climb back to sanity. By April and May, we hit what most consider the absolute best window for being in the water.
In April, the marathon fl water temp usually hovers around 78°F. It’s that perfect middle ground. It’s cool enough to wake you up but warm enough that you don't need to gear up. This is also when the visibility at the reefs—like Coffin’s Patch—tends to settle down because the winter winds have stopped stirring up the "muck" from the bottom.
By May, you’re hitting 81°F. At this point, the wetsuits go into the closet. You might wear a rash guard just to keep the jellyfish stings or the sun off your back, but for warmth? You’re good.
Summer: When the Ocean Becomes a Hot Tub
Then comes July and August. This is the part that catches tourists off guard.
By August, the water temperature in Marathon can hit 87°F or higher. In recent years, we’ve seen extreme spikes. In July 2023, a buoy in nearby Manatee Bay recorded a staggering 101.1°F. While that was an outlier in a very shallow area, it’s a reminder that the "cooling" ocean isn't always cooling in the summer.
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When the water hits 88°F, swimming doesn't feel refreshing. It feels like you’re sitting in a giant, salty bowl of soup. This heat is also a massive stressor for the coral. If you’re diving during a heat spike, you might notice the coral looking a bit pale—that’s the beginning of bleaching.
- June: ~83°F
- July: ~85°F
- August: ~87°F (The "Hot Tub" phase)
- September: ~85°F
For fishermen, this heat changes everything. The big logic is simple: fish don't like being boiled. They’ll head for deeper, cooler water or hide under the bridges where there’s shade and high-velocity current. If you're chasing Tarpon or Bonefish, you have to hit the flats early in the morning before the sun turns the water into a furnace.
Why the Location Matters: Bay Side vs. Ocean Side
Marathon is unique because it’s a narrow strip of islands. You have the Gulf of Mexico/Florida Bay on one side and the Atlantic on the other.
The Bay is shallow. Like, really shallow. Because there’s less volume of water, it reacts to the sun and air temperature much faster. On a hot July afternoon, the Bay side might be 3-4 degrees warmer than the Atlantic side. Conversely, in the dead of winter, a cold snap will chill the Bay much faster than the deep Atlantic.
If you’re looking for the most stable marathon fl water temp, head out to the reef. The proximity to the Gulf Stream acts like a thermostat, keeping the ocean side from swinging as wildly as the back country.
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Fishing and Diving: The Temperature Guide
Temperature isn't just about your comfort; it’s about the biology of the Keys.
- The Snook Factor: Snook are notoriously sensitive. If the water drops below 60°F (which is rare but happens during extreme cold fronts), they can go into "cold stun." They basically stop moving.
- The Mahi Run: When the water starts warming in the spring, that’s your signal. Mahi-mahi (Dolphin) start moving through the Florida Straits as the "blue water" pushes in.
- The Lobster Crawl: When the first real cold fronts hit in the fall (dropping temps into the 70s), it often triggers the lobsters to start moving.
What to Pack Based on the Month
Don't trust the hotel pool temperature. Most of those are heated to a consistent 84°F. The ocean is a different beast.
- Jan–March: Bring a 3mm or 5mm wetsuit. Even if you think you’re tough, the wind on the boat ride back will change your mind.
- April–June: A 1mm skin or just a swimsuit. This is the prime time.
- July–September: No thermal protection needed. Focus on UV protection because the sun reflection off the water is brutal.
- October–December: Watch the forecast. If a front just passed, the water will be choppy and cooling.
What Most People Get Wrong
The biggest misconception is that "South Florida is always hot." I’ve seen people show up in February with nothing but board shorts and wonder why they’re miserable on a snorkel boat.
The water temp doesn't just change by the month; it changes by the tide. An incoming tide brings in fresh, stable-temperature water from the open ocean. An outgoing tide pulls water off the sun-baked (or winter-chilled) flats. If you're planning a long swim, check the tide. It can change the "feel" of the water by several degrees in just an hour.
Actionable Steps for Your Trip
Before you zip up your suitcase, do these three things:
Check the NOAA Vaca Key Station (VCAF1). It gives you real-time data, not a "monthly average" that might be three weeks out of date. It’s the gold standard for anyone who actually works on the water in Marathon.
If you’re visiting in the "shoulder" months (November or March), call a local dive shop like Captain Hook’s or Abyss Dive Center. Ask the crew what they’re wearing. If the divemasters are in full suits, you probably should be too.
Understand that marathon fl water temp isn't just a number—it’s the pulse of the islands. It dictates where the fish are, how clear the water is, and whether your vacation is spent in the water or just looking at it from the bar.
Keep an eye on the wind direction. A south wind brings the warmth; a north wind brings the chill. Pack accordingly, and you won't be the one shivering on the sand while everyone else is having the time of their lives.