A method and an apparatus for retrofit hydrolization of seawater for production of halogen biocides in situ. A method for effecting an in situ generation of biocide as an aid in anti-biofouling of a device disposed in a volume of salt water includes a) associating a cathode electrode to the device; b) associating an anode electrode to the device with the anode electrode spaced apart from the cathode electrode; and c) hydrolyzing one or more components in the volume of salt water to generate a halogen biocide at the anode electrode with the biocide flowing from the anode electrode away from the cathode electrode as a biocide film, the film responsive to a physical arrangement of the associations of the electrodes with the device.