One does not simply build an International Space Station. It takes years of planning and, for the astronauts charged with its assembly, months of training and Extra-Vehicular Activity (EVA) practice in a simulated micro-gravity environment—that also happens to be the world's largest indoor body of water.
It's known as the Neutral Buoyancy Laboratory (NBL)—a 202 foot-long, 101 foot-wide, 40 foot-deep lagoon located at the Sonny Carter Training Facility in Houston, near the Johnson Space Center. It holds 6.2 million gallons of water, 5,000 gallons of which must be replaced weekly to compensate for evaporative losses alone.
The facility provides would-be astronauts the experience of a micro-gravity climate similar to that of space and allows them to practice EVA missions in a controlled environment. As such, the NBL is equipped with full-size mock-ups of the International Space Station's backbone trellis, the JAXA HTV, the European Space Agency's ATV, the SpaceX Dragon capsule, and the Orbital Sciences Corporation Cygnus.
Trainees, accompanied by a pair of safety divers, are lowered suit and all into the pool by way of an overhead crane and are then sufficiently weighted to achieve neutral buoyancy. Water does exert a much higher degree of drag—tools tend to stay where you leave them underwater rather than float away—so the simulation isn't perfect. Even so, trainees may spend up to six hours at a time practicing their spacewalking moves.