A water-soluble coating composition for ship bottom comprising(A) 70 - 95 parts by weight of a vinyl copolymer of(a) 5 - 40% by weight of monoethylenic unsaturated monomer having at least one basic nitrogen atom,(b) 5 - 40% by weight of monoethylenic unsaturated monomer having at least one amide group, and(c) at least one monoethylenic unsaturated monomer copolymerizable with the said components (a) and (b), a total of the components (a), (b) and (c) being 100% by weight, and(B) 30 - 5 parts by weight of a water-soluble epoxy compound, shows excellent effect on controlling release rate of the toxic material from the primer coating layer and on inhibiting adhesion of marine organisms to ship bottoms, when applied to the primer coating layer of ship bottoms.