Monopolar DC transmission line: why is the preferred polarity negative and water used return path?
How we will palace wire in water?

Buy a long stainless steel pipe. Insert pipe in earth so that it makes good contact with water in the ground. Done. 

There is a problem with using earth as a conductor because the current will displace all kinds of metals and minerals dissolved in the water. This effect might be less of a problem when the earth is the positive lead instead of the negative.

Also see this site where it states: The monopolar link is not much in use nowadays. In my opinion it has serious flaws and that is the reason for this.

I do not see a valid reason why a negative voltage would be better for overhead lines in regard to radio interference. It is also not clear if the book means negative DC, in which case RF interference cannot happen as DC cannot interfere with RF. Or that the book means negative on AC overhead lines which is wrong as well as AC has no negative.