Water, Water, and many drops to drink
Omizu (mizu if you're not using the honorific "O") is the Japanese word for water. If you're visiting Japan you can be assured that the water from taps is safe to drink wherever you're visiting. However, if you really insist on paying 100 to 200 yen for a bottle of water you'll find plenty of brands in any convenience store, supermarkets, or vending machines. You can buy plain water, flavored water, energized water, and of course, sparkling water.
Per capita consumption of bottled water in Japan is still relatively low, at about 20 liters per year per person, as compared to many other countries. For example, it's about 1/5th of the US consumption.
In the onsen (hot springs bath) the water is usually rated for the amount of various minerals in it and the ills and aches those minerals in the onsen's water is supposed to alleviate. Anything from curing your dry skin problem to making your arthritis feel better. Of course, the fact that you're soaking in 40C (about 104F) temperature water may have something to do with that healing effect too.
Water is also used for purifying. When visiting a temple or shrine stop by the area that has water flowing and a few dippers. Use the water to wash your hands, then put a handful in your mouth and rinse and spit it out; this purifies you for your visit to the temple / shrine.
Water is generally pretty abundant in Japan, but I do remember driving along a road in Saitama Prefecture this summer where the flashing roadside sign alerted one and all to the fact that this area was suffering from a "mizu-pinchi" (water pinch, or water shortage).