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 60m long, 30m wide, 40m deep lagoon located at the Sonny Carter Training Facility in Houston, near the Johnson Space Center. It holds 23 million litres of water, and 19,000 litres of that 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 practise EVA missions in a controlled environment. As such, the NBL is equipped with full-size mockups 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 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 practising their space-walking moves.