industry is not like the Merchant Marine
.... You can't just show up at the union hall with a Seaman's card and wait for work... You don't need any certifications to work fishing
boats as a crewman.
If you are set on Commercial
Find out were the local commercial boats are in your area and start walking the docks... A good time to show up is a couple hours before they are leaving, since their own crewman fail to show on occasion.
Once you have worked a couple of boats, you will gain a reputation and other Captains will hear about you.
If you would rather work in the Commercial Maritime field... There are a couple of ways to go....
If you do not want to go to the Maritime Academies (College) then I would suggest contacting the Inland Boatman's Union (IBU) Southern California
Region in San Pedro. They can provide information on how to get your USCG Certifications and Union Card to work as a Merchant Marine
Here is there link: https://www.ilwu.org/contact/629-2/