Floating docks can become damaged in a number of ways. Ice, boats and even the encasement system can harm the barrels that keep the dock afloat. Often a float will start taking on water, causing the dock to list. You can repair many floats by simply injecting air into them to blow water out.
Remove the damaged dock float from your dock and take it to a workbench. Empty the water from it and allow the crack to dry.
Place the high density polythene stick into the hot glue gun. Turn the gun on. Once the plastic starts melting, apply it to the crack in the float. Allow the glue to dry. Place the float back in the water.
Locate the filling cap on the float. It should be located on the outside of the dock for easy access. Rotate the float so that the cap is above the surface of the water if necessary.
Open the filling cap. It should be either a threaded cap or a lift-off style cap. Fill it with enough water to slide it under your dock.
Place two levels on the dock perpendicular to each other. They should be in the corner of the dock that is sinking. Make sure they are not too close to the edge so that they won't fall off.
Connect the air pump to the cap. Turn the air pump on to start injecting air into the float. Keep an eye on the levels, and when they are both close to level, turn the pump off. Remove the filling hose and replace the cap.