I did a calculation the other day to estimate how many nuclear plants it would take to power a system to pump and desalinate ocean water to replace the flow of the colorado river. My wild gues was about 10 which of course would likely not be feasible to build on any helpfful time scale. The math said about 1200 though! So that won't work.
And forget about wind or solar for that scale!
Recently, for a lark I sat down to figure out if it would be possible for me to make enough fresh water by reverse osmosis (RO) to meet my household usage. It was possible by using solar energy, but I am nowhere near the ocean to even get some of that salty water to process.
Current technology allows desalination at the cost of 3-5.5 kWh/cubic meter. A cubic meter is 1,000 liters, or 264 gallons. The average household consumption is 378 liters or 100 gal/day/person. Hence, for 2 of us the required electricity power is 2.3-4.2 kWh/day. I can generate that energy with solar panels without too much trouble. Again, the problem is getting the seawater where I am.
By the way, not just large ships but ocean-going leisure crafts generate fresh water by desalination. The source of power is often a diesel generator (the produced water is expensive), although some boaters have solar panels.
Israel desalinates water at the cost of US$0.40/cubic meter. That agrees with the 3-5.5 kWh/cubic meter estimate above. Singapore experiences about the same cost.
Supplying all US domestic water by desalination would increase domestic energy consumption by around 10%, about the amount of energy used by domestic refrigerators. Domestic consumption is a relatively small fraction of the total water usage.
Desalination is feasible, but the water cost while barely affordable for household consumption is too costly for agriculture. Here in the US, California had some desalination plants built during a drought, but then mothballed them when the rain returned.
I will explore the idea of large-scale desalination further in the next post. And I will try to tie it to nuclear energy to stay within the thread topic.