The official 3- and 12-nmi lines are demarcated on the highest-resolution NOAA charts for a particular area. These charts can be hard to find on-line, though it is possible to find certain areas though various state GIS websites and such. I also think the NOAA is systematically making vector data of the lines available.
In the case of Catalina Island, it has it's own 12-nmi belt of territorial sea, but the space between it and the mainland (so long as it is at least 12 nmi from either shore) is International waters.
There is a belt extending 24-nmi from shore called the "Contiguous Zone", in which a nation may exercize authority mainly to enforce environment and customs regulations. This area is still considered Internation waters, however.