Bay seniors interested in a summer vacation on a mountain lake with their families in tow might want to consider the easy going (10 mph) pleasures of houseboating. In nearby Napa County Lake Berryessa offers 165 miles of shoreline and cool waters that are a sweet relief from Valley heat. Boats that sleep 7 start at around $480 a day. Further north glorious Shasta Lake has 370 miles of shoreline. Boats that sleep 14 go for around $3600 for 3 nights. All rental companies offer a 30-60 minute course in navigation and safety before you ever take the wheel.