I don't think the site selection criteria were described in the book, but the Martian trilogy is based on a lot of science and discussions with scientists working on Mars mission plans, so we can think about why would we choose this site to answer why the First Hundred went there.
From the map at http://fwb.home.xs4all.nl/rgbmars.html, we can see that Underhill is almost bang on top of the equator.
There are two main advantages to being on the equator:
- Less seasonal variation - Mars is harsh, but Martian winters are harsher
- Lower energy requirement to achieve orbit - okay, so the First Hundred are not looking to launch back right after they get there, but it makes sense to plan ahead
It seems water isn't too hard to find on Mars (in the books) - there's enough stored in aquifers to flood the northern basin. In fact, that may be another reason - if there was a plan to flood the northern basin, any settlement would have to be relocated, and that would be huge cost in other resources.
I guess they could have started near the southern polar region - but if I recall correctly, Southern winters are even harsher?
Underhill is also sited near two of the most spectacular formations - the Mons volcanoes, and Vallis Marineris.
Finally, there's all the equipment - I'm not sure how easy it is to land unpiloted containers away from the equator - but it would require additional fuel to get the inclination for a polar orbit. Because of the rocket equation, more fuel at Mars means a lot more fuel to get to Mars, which means a lot, lot more fuel to get into Earth orbit.
So, taken altogether, it was probably a balance of advantages and disadvantages which weighed the decision to site Underhill where they did.