After they're done getting a space shuttle into the upper atmosphere, the solid rocket boosters drop off and fall into the ocean. They're recovered by the NASA vessels Liberty Star and Freedom Star and refurbished for additional use. This video shows how it's done. It's rather long, so let me point out some highlights:
0:28 -- Dolphins!
1:41 -- Liberty Star arrives at the location of a rocket.
2:30 -- Divers attach towing rig to the rocket.
3:20 -- Liberty Star begins towing the rocket.
5:00 -- Freedom Star gets underway.
5:45 -- Freedom Star crew watches shuttle liftoff.
6:25 -- Freedom Star arrives at the location of the other rocket.
7:40 -- Rocket flops over in the water.
8:05 -- Freedom Star enters the harbor.