Life on Earth depends on water: the unique chemistry of water allows life's chemical reactions to occur, and living things are mostly made of water, based on weight. Also, life is thought to have started in the water, so looking for water first would point to any place on Mars where life could exist and would be most likely to exist. This would save time and resources in searching for life on Mars by eliminating many impossible or unlikely dry sites.
Work Step by Step
Much of this chapter deals directly or indirectly with water and things that interact with it. Given that central role of water for life, it makes sense to look for it first. If no water is found, no life is likely to be found, at least not any life familiar to us on Earth. Finding water means finding potential places for life to exist on Mars.