I found a great deal of KSR's trilogy boring, but kept reading because of stretches I really enjoyed. I prefer Niven, Asimov, Hogan, Bova, and Brin over KSR by far.

As far as water on Mars goes the idea that Mars had a lot of frozen water buried was generally expected. The amount has surpassed expectation by being at or surpassing the upper limits of estimates, but even lower range estimates would have made Mars an attractive target. The use of geothermal power was discussed as a highly desirable possibility not a necessity on pages 208-211 in "The Case fo Mars". Nuclear (pages 205-206) and solar (206-208) power were clearly discussed as an alternatives. The basis for surmising that geothermal power would be available was quite clear IMO. The fact that it is clearer now does not mean it was an outlandish guess then. It was also pointed out that this source of power is definitely not available on the Moon or asteroids. Page 211 clearly states colony growth rate is dependent on power and solar and wind power are key to growth with geothermal power being a much stronger source of power. "The more power it has, the faster it will grow; the faster it grows, the more power it will have. Once it is possible to produce, solar, wind, and especially geothermal power on Mars, the growth of the base will become exponential".

What you may not understand is that the low surface pressure on Mars would make even room temperature water on Mars a source of geothermal power. You don't need Icelandic style vulcanism to get geothermal power on Mars. Water boils at 5 mmHg at only 1.2C and 11.2C at 10 mmHg. Since Mars is a terrestrial planet with similar composition it should still be generating about the same amount of heat per unit mass from nuclear decay as the Earth. The gradient is different because of a difference in surface to mass ratio. Without a doubt Mars gets hotter as you drill down just as the Earth does. The only question is does the water layer reach the appropriate temperature to boil and it is not a stretch to guess it does in at least a few places.

Regards,
Mark