Cal. Harbors and Navigation Code § 652.5

(a) The use of a distinctive blue light as prescribed by the department is reserved for public safety vessels and may be displayed during the day or night whenever the vessel may be engaged in direct law enforcement activities, including, but not limited to, those activities specified in subdivision (a) of Section 663.7, or public safety activities conducted by a fire department or a fire protection district, where identification of a public safety vessel is desirable or where necessary for safety reasons. (b) That light when used shall be in addition to prescribed lights and day signals required by law. (c) The display of these blue lights on vessels for other purposes is prohibited. (d) A vessel approaching, overtaking, being approached, or being overtaken by a moving public safety vessel operating with a siren or an illuminated blue light, or a vessel approaching a stationary public safety vessel displaying an illuminated blue light, shall immediately slow to a speed sufficient to maintain steerage only, shall alter its course, within its ability, so as not to inhibit or interfere with the operation of the public safety vessel, and shall proceed, unless otherwise directed by the operator of the public safety vessel, at the reduced speed until beyond the area of operation of the public safety vessel. (e) The operator of a cable ferry shall take whatever reasonable action is necessary to provide a clear course for a public safety vessel operating with a siren or an illuminated blue light, or both. (f) For purposes of this section, “public safety vessel” means a law enforcement, a fire department, or a fire protection district vessel.