OK, I found it.
In the California Marine Fishing Regulations (read SALT water)

Item 28.65:

(d) No gaff hook shall be used to take or assist in landing any fin fish shorter than the minimum size limit. … No person shall take fin fish from any boat or other floating device in ocean waters without having a landing net in possession or available for immediate use to assist in landing undersize fish of species having minimum size limits; the opening of any such landing net shall be not less than eighteen inches in diameter.