How does the saguaro cactus store water?

As is the case with all living organisms, a cactus plant can't live without water. Specifically, it needs water to take in the nutrients that it needs to live. Minerals and trace elements in the soil only can be taken in dissolved form by a plant's roots. No water means no nutrients.

Additionally, the dissolved nutrients are needed for the photosynthetic interaction with sunlight. Sunlight and dissolved nutrients yield energizing photosynthetic products such as starches and sugars. So no nutrients means no photosynthetic products and therefore no life sustaining activities within a cactus plant.