Field testing to determine surface preparation requirements for underwater coating showed that surface preparation by abrasive blasting is best, but that power wirebrushing or waterblasting may provide satisfactory surfaces for coating underwater. All of the formulations containing organotin biocides displayed some degree of resistance to marine fouling and those with 6-percent tin compared quite favorably with the standard Navy copper-based antifouling paint used for comparison purposes. The new formulations can be used to repair damaged coatings or restore fouling resistance on waterfront and offshore structures and on hulls of ships without their removal or drydocking, thus resulting in considerable savings in time or money.