To follow up on what Batman says, what you use to find the leak depends a lot on the circumstances.
If you get a flat by the side of the road (and you don't have a spare tube) then you obviously can't use the tub of water (unless you find a convenient pothole filled with rainwater). In other circumstances the water tub (or bathroom sink or whatever) is reasonably practical, but it may not be necessary.
First consider the circumstance. You're riding along and suddenly hear "Pfft, hiss, hiss, hiss..." as the tire rapidly goes flat. You can't ride home on the rim, and your spouse can't come and get you, so you need to fix the flat.
Even if you have a spare tube you should find the leak and check the tire at that spot for something embedded (or else your spare tube may be holed as soon as you inflate it). So you remove the tube, keep track of which side is which (if possible) and find the hole. Usually the way to do this is to inflate the tube with your pump and loop it over your head, as you rotate it around, listening for the leak. When you hear it, hold the suspect portion of the tube near your face to feel the wind blowing on it, to zero in on the hole.
Having found the hole (if possible, marking it with pen or chalk), you then lay the tube on the tire, aligning the valve stem with the valve hole in the rim (here is where it's nice to know which side is up, so you don't have to check both ways), and then check the tire in the vicinity of the hole for any embedded wire, nail, glass, or thorn.
Sometimes, though, a leak is hard to find by hearing/feeling it, either when it's too slow, or when it's so fast that the tire won't hold air long enough to listen/feel for it. In these cases a water tub is the way to go. The water tub is also handy for discovering valve leaks.