"The two solid rocket booster recovery ships, Freedom Star and Liberty Star, are stationed in the Atlantic Ocean about 140 miles northeast of the Kennedy Space Center (off the coast of Jacksonville, Fla.) in their mission station position. They were deployed on September 5 and arrived the next day.
"The booster recovery operation takes about six hours. The boosters are plugged, and compressed air is pumped into the interior, a procedure known as dewatering. This changes their position in the ocean from vertical and bobbing like buoys, to a horizontal position resembling logs in the water. This allows them to be towed back to Port Canaveral.
"After the ships retrieve the boosters, they are returned to the port to be prepared for shipment by train to Utah where they are readied for a future shuttle launch."
Mission STS-115 on NASA's Launch Blog