How Hard Is it to Convert Seawater Into Fresh Drinking Water?

The answer is about 3.7kWh per tonne for large installations. Has any reader been to Cyprus?

Israel and other coastal countries have tried this for a long time but it is very expensive. Salt water corrodes and everything must be boiled and cooled

Not being a physicist, I'm not sure about this but I read somewhere that many particles in water can be give a magnetic charge, thus making them removable by attraction to the opposite pole. Could this concept be applied to salt water to make the process more economical or are there too many non-chargeable particles for it to work?

This is an area of much research. As one poster noted, it is the corrosive nature of sea water that gives a great deal of the problems. Still, there are two main ways to desalinate sea water (I'm not sure about electric or magnetic methods, and in the literature I've not come across this.) One method is to distill the water; essentially doing what nature does. There are many small glass boxes that people sell to do this, but they are incredibly inefficient both in their optics and energy management. Many solar based distillers have been tried, and functioned. But the energy needed for the water for a small city is in the MW of peak power which becomes about a 1km^2 area of collectors (to give about 500-1000L/s, Ensenada 400 000 people uses 700L/s; I can get better number if needed.) This is feasible, but expensive (and still my personal favorite as it can also yield electricity from the steam produced.) The other way is reverse osmosis; basically a very fine filter. These systems are smaller then a distiller, but require huge pressures and usually have a MW power plant to supply them with power. Israel uses many of these, and here is Baja California they want to install one. I dislike greatly this idea for two main reasons; 1-they put back to the ocean large quantities of extra salty water, 2-the power plant, here our power is mainly natural gas plants. Israel is money rich and land poor, but Baja (and most of the Americas) are money poor and land rich. This leads back to a solar system, but to go from sun to electricity to pressure to fresh water I think would consume more energy then sun to heat to steam to water. Many systems for solar distillers also could be more efficient if they had heat exchangers, warming the incoming sea water with the outgoing fresh water steam. I hope to make a small (home/business/closed community ) system using a heliostat array and distiller system, but that needs to wait for me to start the open-source heliostat array system first. There are publications of PV reverse osmosis systems and Solar distillers from the 1970's and 1980's. I'm not sure why large scale systems have not been made, but the distiller plants have gone out of use largely due to lack of maintenance.