Bay seniors feeling the urge to swim but shudder at the thought of dipping into the 60 degree Pacific have all kinds of options in nearby (and 5 to 10 degrees warmer) lakes and rivers. In the Delta the Sacramento River has dozens of sandy beaches just an hour from the city. On the Russian River Monte Rio Beach is a gleaming stretch amid the Redwoods, though caution is advised as downstream lurk some hazardous currents. Just off Highway 24 before the Caldecott tunnel Lake Temescal is a refreshing 20 minutes from town. Just because you are old and retired enough to swim during the week doesn’t mean you’ll be alone. Warm weather almost guarantees kid crowds.