Don, long ago there was a thread about using marine currents and tides. Discussed were two projects, one British, in the North Sea and the other in Maritime Canada using tides. I don't know wht progress has been made recently, but I was going to check in google. You may be familiar with the efforts around Great Britain.

Your other suggestion is, of course, experimentally demonstrable, but against it, very large land areas would have to be used. The daily temperature variation would have to be considerable.
In the U.S., even the southern "deserts" have spells in the winter when the temperature has little variation, sometimes snow and freezing. Moreover, the deserts are loved by many of us; we hate to see them torn up or disturbed. Also unfortunately, cities like Phoenix have grown exponentially and are demanding more water than is sustainable. There is no "unwanted real estate" left (perhaps in the Sahara?).

I won't comment on the BPA; that is a digression. Also TVA, etc.

Rumcoke, the Mohave project in California heats liquid steam in tubes; the technique perhaps is practical for individual users as well.