These public swimming pools are normally only open during the summer months. 

Veterans Home of California
100 California Dr, Yountville
(707) 944-4541
(707) 944-4908 direct pool line
Open May/June/July: Tuesdays-Sundays 1pm-4pm
Open August: Sat/Sunday 1pm-4pm
$4 Adults/$3 Children

Calistoga Community Pool
1745 Washington St.
Calistoga, CA 94515
(707) 942-2846 or www.ci.calistoga.ca.us 
Spring: Rec Swim F 3:30pm-7:30pm, Sat/Sun 1pm-5pm
Summer Hours: Rec Swim M-Th 1pm-5:15, F-Su 1pm-7:30pm 
Fall Hours: Rec Swim F 3:30pm-7:30pm, Sat/Sun 1pm-5pm
$6 Adults/$5 Seniors/$3 Children

Napa High School
2475 Jefferson Street, Napa
Season: June 25-August 16
Tuesday-Friday, 2:00-5:00PM
Saturday, 1:00-4:00 PM
Closed July 4!
Rates: 18 & Younger ($2), 19+ ($3.50), Seniors ($2), Spectators: Street Clothes ($1) 

St. Helena High School
(707) 967-2740
1401 Grayson Avenue
St. Helena, CA 94574
 
St. Helena Community Pool
St Helena, CA 94574
(707) 963-7946

Bothe Napa Valley State Park
3801 North Saint Helena Highway (Highway 29)
Calistoga CA 94515
(707) 942-4575
Open year round
$8 day pass/car, then $5 Adult/$2 Children 6-17/Free Children under 5

If you are looking for protected swimming areas in lakes or coves:

Lake Berryessa Visitors Center
5520 Berryessa-Knoxville Rd
Napa, CA 94558
707-966-2111
or 
Lake Berryessa Recreation Guide
www.berryessatrails.org
707-966-5211