From the Folsom Lake Web Site
Yes you can drink on the lake in your boat!!http://www.folsomlakemarina.com/alcohol.html
New Alcoholic Beverage Policy
Planned for Folsom Lake, Lake Natoma
In an effort to enchance public safety and combat increasing law enforcement problems, California State Parks will prohibit consumption of alcoholic beverages in all day use areas at Folsom Lake and Lake Natoma, effective March 1, 2003.
This policy prohibits the consumption of alcohol or possession of an open continer of alcohol in any day use and lake surface areas of the park.
Exceptions to the policy are:
Registered campers within a designated campground
Within a vessel Floating on the lake surface, but not touching shore
Permitted Special Events in designated areas.
An open container is defined as any bottle, can or other receptacle containing any alcoholic which has been opened, or seal broken, or the contents which have been partially removed.