Harbors and Navigation Code § 6243

Each commissioner shall, within ten days after his appointment and before entering upon the discharge of the duties of his office, take and subscribe to an oath or affirmation before an officer authorized by law to administer oaths, that he will support the Constitution of the United States and the Constitution of the State of California, and will faithfully discharge the duties of his office according to the best of his ability. The oaths or affirmations shall be filed in the office of the district.