Plan your Alcatraz or San Francisco Bay open water swim with 18 years of water temperature data and real seasonal trends. Know what to expect and how to prepare.
No, there aren’t icebergs in SF. Yes, it’s still cold, but that’s what wetsuits are for, if you need one.
Swimming from Alcatraz, crossing under the Golden Gate, or tackling any stretch of San Francisco Bay? The water’s colder than most people think. Knowing the temperature, and how it changes, can make or break your swim.
We’ve summarized 18 years of daily 7 AM water temperature data from NOAA, recorded just inside the Golden Gate Bridge, and paired it with our year-round experience swimming in these waters to help you plan smart.
Whether you’re trying to pick the right season for your Alcatraz swim or wondering how cold is too cold, this guide cuts through the noise. You’ll get real numbers, monthly averages, seasonal trends, and honest insight into what cold water actually feels like.
San Francisco Bay Monthly Water Temperature
7AM (±1hr) Average Water Temperature at NOAA Station 9414290
Temperature Scale
Detailed Average Water Temperature Chart for San Francisco Bay
Historical average temperatures measured at 7:00 AM on the first and fifteenth days of each month are displayed here.
It’s important to note that water temperature in the Bay shifts daily with the tides, weather, and wind. In winter, swimmers often see 1–2°F changes over a few hours as the tides change. From summer through fall, shifts of 4°F or more are common. These patterns are typical in the central Bay—between the Golden Gate Bridge and Alcatraz—where currents and wind drive constant variation.
Planning for Open Water Swimming in San Francisco
At Pacific Open Water Swim Co., we swim year-round, day and night, in all seasons. No wetsuits, just skin. Winter lows typically range from 49 to 50°F and can feel brutal at first, but the sting fades quickly. A few degrees warmer makes a world of difference, and by late summer and fall, the Bay often reaches an absolutely balmy temperature of 60 to 62°F or more.
That said, we don’t care if you wear a wetsuit or not. Many swimmers do, and for good reason. Wetsuits add warmth, safety, and confidence, especially when you’re building cold water experience. There’s no judgment here. We’re here to help you swim smart and finish strong, whether it’s your first Alcatraz, a Bridge to Bridge, or one of our sixty-plus other courses. (No surfing or diving wetsuits though. They’re a real challenge to swim in.)
If you’re new to swimming in San Francisco Bay, aim for fall through mid-November. The water is at its warmest, and conditions are ideal. With a proper swim wetsuit, April through November is totally doable.
