What can one do with shower water?
The search term you are looking for is probably "greywater" (or graywater.) Basically non-toilet (watercloset) waste water.

There are two aspects - one is legal and varies by location, the other is practical and does not, so much.

The legal aspect essentially comes down to whether your local jurisdiction prohibits or allows use of graywater (rather than requiring that it go into the sewer), and any specific regulations/rules you must follow to use it in the jurisdiction. Some places essentially limit you to the output of the kitchen sink, for fear of tiny amounts of bad things that might be in laundry or shower water; others do not. Most have fairly strict rules to prevent the cross connection of sewage and greywater lines.

The  practical aspects tend to have to do with things like filtering it through a reed bed or similar wetland plant-filter before making further use of it, which deals with the soaps, toothpaste, etc. Any practical permaculture book tends to have a few diagrams of this sort of thing in it. How practical this for a given house/garden depends on available space and topography.

If you are not in a terribly dry region, storing rainwater may be easier to manage than diverting and filtering greywater.