How Cold Is It? SF Water Temps for Swimmers
Alcatraz and San Francisco Swimming Water Temperatures: What to Expect by Season, with Wetsuit Tips
What You Need to Know in Three Lines
Winter and spring: 50–54°F. Summer: 54–59°F. Fall: 58–62°F.
The Bay stays cold year-round—with late summer offering the warmest window.
Wetsuits help. Skin swimmers need prep, acclimation, and respect for the cold.
Why Water Temperature Matters More Than You Think
If you plan to swim Alcatraz—or anywhere in San Francisco Bay—know what to expect from the water temperature. It’s a defining part of the experience, and one of the few variables you can’t control.
It’s also the biggest factor in how your swim will feel, how long you can stay in, and whether you’ll need a wetsuit—and unless you’re fully acclimated, you will. You’ll be rewarded for your mental and physical preparation, showing up fully, and whatever success means to you.
The Bay is cold year-round. Even in summer, temperatures hover in the mid to high 50s°F, with only brief climbs into the low 60s°F. That can shock the system if you’re not acclimated—but it’s also why the Bay is one of the best training environments in the world for marathon and ultramarathon swimmers.
We’ve logged thousands of swims here. We swim year-round. We have the data—and the numb fingers—to back it up.
San Francisco Bay Water Temperatures
(2010–2024)
From January through December, water temperatures in the central San Francisco Bay—including near Alcatraz—typically stay in the 50s°F. For a month or two each year, temps may reach the low to mid 60s°F, mostly in late August through September. In recent years, we’ve even seen upper 50s°F holding into November.
This 14-year dataset is based on NOAA sensor readings near the Golden Gate. The sensor stopped reporting in 2024, but its data aligns with what we see daily—from the boat and in the water. We trust it. You can too.
How Much Does It Vary?
San Francisco Bay water temperature isn’t static. It shifts with tides, wind, and current—sometimes during a single swim. In winter, expect 1–2°F swings. By late summer, 3–4°F changes are common between the Golden Gate and Alcatraz. Swims north toward the Richmond–San Rafael Bridge or south past the Bay Bridge, often seeing even greater variation—5°F or more.
Translation: You won’t always know the exact number until you’re in the water. But the chart above gives you a reliable baseline.
Do You Need a Wetsuit?
Short answer: Yes—unless you’ve trained for the cold.
At Pacific Open Water SwimCo., we swim skin year-round (no wetsuit). That takes time, exposure, and consistent cold water adaptation—a topic we’ll cover another time.
For most swimmers, especially first-timers, a triathlon wetsuit is the smart move. It provides warmth, buoyancy, and no drag.
What type of wetsuit for an Alcatraz swim?
First, get a wetsuit for triathlons or open-water swimming, not surfing suits or diving wetsuits. It will make your swim much easier!
Wetsuits for triathlons or open water swimming:
Made from closed-cell neoprene
Slick outer skin allowing you to skim through the water with less resistance
Thickness varies, providing more buoyancy in the legs and less in the arms for better movement
Surf and dive suits on the other hand…
Made from open-cell neoprene, it allows more water in the suit (heavy!)
Rougher exterior creating resistance in the water (slows you down!)
It tends to be thicker, but that doesn’t mean warmer if you’re weighted down and in the water longer (cumbersome!)
In short, we strongly advise against surfing or diving wetsuits. They are too bulky, lack flexibility, and are not designed for efficient swimming.
Full-Sleeve, Sleeveless, Shorty…
Again, it’s personal preference, acclimation, and comfort but a good guide -
• 50–55°F → Full-sleeve
• 56–62°F → Full-sleeve or sleeveless
• 62°F+ → Optional, based on comfort and distance
Renting a Wetsuit in San Francisco?
Sports Basement – Presidio is the only reliable wetsuit rental option in town—and they’re swimmer-friendly. As of March 2025:
Rental pricing: $30–$55/day
Blueseventy Fusion (Full Sleeve)
5mm legs, 3mm torso, 2mm armsBlueseventy Reaction (Full Sleeve or Sleeveless)
Warmer, more performance-focused fit
If you’ve never worn a wetsuit in the ocean, test the fit with a short swim. Comfort matters more than specs—or marketing.
Remember to prepare for chafing, especially around the neck and underarms. A rash guard or products like Body Glide, TRISLIDE, or SBR Skin Slick can make a big difference.
Best Time of Year to Swim Alcatraz?
Late August through early November.
That’s when water temps peak, winds are lighter, and tide swings smooth out. If you’ve got the flexibility, that’s your window.
But with a wetsuit, the Bay is swim-ready all year. Don’t let water temperature be the reason you wait.
We swim in every season—rain or shine. Whether you’re going skin or suiting up, knowing what to expect helps you stay safe, fast, and focused.
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