An anti-fouling or fouling release condensation curing organopolysiloxane composition is provided comprising (A) a diorganopolysiloxane having at least two silicon atom-bonded hydroxyl and/or hydrolyzable groups in a molecule, (B) a silane having at least two hydrolyzable groups in a molecule and/or a partial hydrolytic condensate thereof, and (C) an ionic liquid, typically a fused salt. The composition cures to an underwater structure to form a coating which is non-toxic, non-detrimental to the environment, and effective for preventing aquatic organisms from depositing and growing thereon over a long term.