I'm not usually the sort of person who participates in extreme sports, unless you count marathon sessions of reading novels. So when a friend said she had "kind of a crazy idea" for something to do on a bright, fogless September weekend in San Francisco, I was thinking, "Hmmm. Wonder where she wants to go for lunch?"

San Francisco's restaurants are well known, but my friend had something else in mind: a trip to the Dolphin Club, which offers open-water swimming--without lifeguards or guides--in San Francisco Bay. Although countless visitors admire the view of the bay from the city, few realize that you can get a unique perspective of the city from the bay--and now is the best time to do it. San Francisco autumns are usually warm and sunny (despite what those foggy Julys might lead you to believe), and the bay water hits its yearly high temperature, a chilly 60 degrees.

The Dolphin Club has existed since 1877 for hardy souls (now about 900 members) who swim in the bay year-round. It maintains a quarter-mile course in Aquatic Park, a cove near Fisherman's Wharf that is protected by a breakwater from such distractions as boats and strong currents. Club members can participate in long swims, including one from Alcatraz (when contemplating this activity, it's best not to recall that the whole point of putting a prison on Alcatraz was so inmates couldn't escape).

We headed down to the Dolphin Club, which is open to the public, at about 11 on a Saturday morning. As if to intimidate us as much as possible, a group of intrepid swimmers seemed to be wrapping up an Alcatraz sojourn just as my friend and I were inching our way to the shore of Aquatic Park. We had changed in the club's sunny locker room, nervously observing that many lockers had padded swimming bonnets hanging off of them--and some had wet suits to boot.

A goose bump-inducing breeze blew off the water, which looked murkier and less sparkling blue up close. As we hesitated on the sand before wading in, I tried to forget a sentence from the release form absolving the Dolphin Club of any responsibility for dangerously cold water, powerful currents and "treacherous sharks" (as opposed, I idly wondered, to kind and loyal ones?). Suddenly, our lap suits and caps seemed woefully inadequate. But in the sunlight I could see Angel Island and Sausalito straight ahead, with the Golden Gate Bridge on my left. Then a man who looked to be in his late seventies walked by us and dove right in. We had to follow.

Follow we did. Although 60 degrees might not sound so cold, the 38.6-degree difference from body temperature knocked the wind right out of me, and I almost forgot to swim as I dog-paddled my way into the cove.

I then concentrated on my backstroke, moving as briskly as I could and catching glimpses of the shore and distant sailboats as I turned. I was thinking so hard about keeping the horrendously bad-tasting water out of my mouth that I almost forgot about the cold. After a while, it just felt good; the combination of the gorgeous setting, bright sun and cold water had an exhilarating effect, and we stayed in for about 45 minutes.

Of course, I've never encountered a mop of seaweed as big as my head--or any marine life, treacherous or otherwise--in my neighborhood pool. On the other hand, in a pool I've never stopped to tread water so I could take in a 360-degree view encompassing the hills of downtown San Francisco, the opposite shores of the bay and Alcatraz.

And no other morning swim has ever made me feel quite so confident that I deserved an indulgent lunch in a trendy San Francisco restaurant.

The Dolphin Club (502 Jefferson St., Aquatic Park, 415-441-9329, www.dolphinclub.org) is open to the public five days a week, Tuesday-Saturday, 10 a.m. to 5 p.m. The $6.50 fee covers use of the locker rooms, showers and sauna. The club is near the terminus of the Hyde Street cable car line and can also be reached via the F-line trolley car on the Embarcadero.