[Mogger]

Where is the water coming from?

It's a closed cycle, Water turned into steam spins the turbine.

The water is condensed, the vacuum produced by doing this provides about half of the power as the steam is sucked through the 2nd half of the turbine.

After being condensed, the water is pumped back into the boiler.

[Equine1952]

Well that’s partially true but you better study up on:
Primary coolant
Condensate
Circulating water

[sit-rep]

and the cooling water for the condensers?