The signs say no alcohol but the web site, the sheriffs and the park rangers all say it is allowed as long as the boat or anchor is not touching the ground.
http://www.folsomlakemarina.com/alcohol.html from the website, This policy prohibits the consumption of alcohol or possession of an open container 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.
Hope this helps, M.