Provided is a marine biodegradation promoter that is a hydrophobic powder composed of a compound in which ionic bonds bond a metal cation having a valence of 2 or higher and two or more monovalent organic anions that have different structures selected from among monovalent organic anions derived from monovalent carboxylic acids, monovalent sulfonic acids, monovalent sulfate esters, and monovalent phosphate esters, said marine biodegradation promoter either dissolving in a 3 mass% aqueous sodium chloride solution, or exhibiting hydrophilicity in a 3 mass% aqueous sodium chloride solution, wherein at least one of the two or more organic anions contains a monovalent hydrocarbon group having 6-30 carbon atoms.