Not to say, cheap, plentiful energy greatly increases water usage by itself.
In Pakistan, every farmer now has powerful immersible pumps with which they suck groundwater dry because of cheap solar panels availability.
Do you need a constant stream fresh water, or just a big initial injection?
> How much loss happens in the system?
For an open cycle system it is 100% obviously.
If it's dangerous for us to drink, it's probably damaging to wherever we dump it.
You will have to condense it from steam. Doable, but not without cost. It's also highly mineralised, and you will have to acid wash hardware frequently.
The reason for the closed loop is to keep mineralisation of the water to a minimum, as otherwise steam tubes and turbines are rapidly fouled.
The open loop loses a significant amount of the initial water load, either through drainage or evaporative losses.
Plants such as The Geysers, north of San Francisco in California, have had output reduced to about 40% of initial generation largely due to groundwater losses.
And if you have freshwater nearby, it will always be more economical.