Cal. Rev. & Tax. Code § 12101

Every insurer transacting the business of ocean marine insurance in this State shall annually pay to the State a tax measured by that portion of the underwriting profit of the insurer from ocean marine insurance written in the United States, which the gross premiums of the insurer from ocean marine insurance written in this State bear to the gross premiums of the insurer from ocean marine insurance written within the United States, at the rate of 5 per cent.