It seems obvious that water has been on Mars at some point, be it long ago or recent.
My question, then, is this: Why are we still looking for evidence of water on Mars? Shouldn't we be focusing on evidence of life instead?
Yes, and we are.
The focus now is on the four ingredients for life, raw materials, energy, water, and favorable conditions. Water is always there, but is not the primary focus. We now seem to be able to find places on Mars where water once flowed using orbital data. It is the job of the surface missions to first confirm that (Curiosity has) and then look for the other stuff. Previous missions have been said to fall under a "Follow the water" strategy. We have now graduated to "Follow the carbon", in particular reduced carbon. That is the main raw material to look for, especially since it is not found easily on Mars -- on the surface and in the atmosphere, all the carbon, and iron as well, is oxidized. Curiosity has already found iron that is reduced, not oxidized, by drilling down a few cm. So it seems to be on the right track.
Note that the search is for past life, not current life. Our ability to find sites on Mars where current life is accessible is limited or non-existent, and even the possibility of extant life is really a long shot. We are far more likely to meet with success if we look for evidence of past life, since we have good evidence that Mars was much like Earth a few billion years ago. Not so much now.
Curiosity is armed with the instrumentation it needs to explore those four ingredients, at least at the scale of a drill sample. Now it just needs to drive to the primary science target in the foothills of Mount Sharp. Stay tuned.
Finding such evidence of life will be highly suggestive, but not conclusive. Unless we get really lucky and a rover finds some gross, obvious morphological evidence of life, another long shot, we will need to bring selected samples to Earth for the much more detailed examinations required to rule out abiotic mechanisms for that evidence.
The Mars 2020 mission is planned to start us on that path, both with more detailed, fine scale searches for evidence of past life in situ, and the collection of selected samples for possible transport to Earth. The report of the Mars 2020 Science Definition Team outlines the overall strategy for the search for life on Mars in great detail. It is highly recommended reading for those who would like an introduction to this topic.