>>14119669A little unfair. You need to separate the heat capacity of liquid water from steam - liquid water does have a very high heat capacity compared to other liquids, steam not as much. The hotter your steam enters the turbine and the cooler it exits, the more useful work can be extracted. So the heat capacity of the steam is more important.
However, its the heat required to vaporize (or condense) water is also important. This is very large for water.
To ensure maximum work extraction, turbines should be operated isentropically (ie constant entropy). What does this mean? It means that amongst other things, you dont want a phase change in your turbine. Liquid settling out on turbine blades is a bad thing.
If your turbine doesnt exchange heat with the environment its in, you can operate at constant entropy. A higher heat of vaporization makes it less likely that your fluid will condense in the turbine.
So for a variety of reasons, the water enthalpy-entropy chart makes steam a good candidate to use in a turbine.
Of course water isnt the only the fluid you can use. But as others have pointed out, its cheap and non-toxic, although high pressure hot water will kill you as quickly as if it were toxic.
