According to the "Nautical Charts App" at the following link (just click the thumbnail map):

http://www.gpsnauticalcharts.com/main/us18022_p1655-san-diego-to-san-francisco-bay-nautical-chart.html

... about 170 nautical miles due west from Los Angeles, the continental shelf drops off abruptly to a depth in excess of 11,500 feet (3500 meters). This is just west of the Rodriguez Seamount (don't know if that means anything to anyone, but it's a landmark nevertheless).

https://en.wikipedia.org/wiki/Rodriguez_Seamount

This is within the 200 nautical mile Exclusive Economic Zone, as described by the Wiki definition entry:

https://en.wikipedia.org/wiki/Exclusive_economic_zone

Also, California would probably love the idea of building a 3.5-km high underwater waterfall as pumped hydro storage of renewable energy from offshore wind farms.