Abstract: A hardenable sealant composition, apparatus for application, and method of implementation are provided for repairing cracks in concrete covered by water. The apparatus of the invention employs a sealant reservoir formed with a high pressure cylinder having twistlock ends. The cylinder is filled with a cement compound that will subsequently harden under water. The cylinder contains a piston and is coupled at one end to a high pressure pneumatic hose. The other end of the cylinder is connected to a material delivery hose which in turn leads through a manually operable metering valve assembly to a hollow injection needle. Holes are drilled into the surface of an underwater concrete structure in which a crack appears, and the injection needle is inserted into the holes, one by one. The sealant composition is formed of a resin, a resin hardener, white cement and underwater cement. It is extruded from the sealant reservoir under the control of the metering valve.