There are two types of water pipes that you should bury. The water pipe from your city's main supply, called the house water supply pipe, and lawn sprinkler pipes. How far down you have to bury them depends on which pipe it is.
The supply pipe for your house is the pipe that runs from the corporation stop--the valve between the municipal water supply and your property's water pipes--to your house. The supply pipe must be buried below the frost line for your area. It must also be buried at least as deep as your building codes require. To find out the code for your area, you should contact your municipality's building permit department or the water company.
Sprinkler pipes do not need to be buried below the frost line for your area. Drain the pipes in the winter to protect them instead. Sprinkler pipes should be buried about 8 in. below the surface to be safe. Keep a map of where they are so you don't accidentally cut through them later.
Call Before You Dig
Before burying either type of pipe, call 811 (in the US) and have all existing buried utilities marked. Don't assume you're safe because you are only digging down 8 in. to install sprinkler pipe. Ground settles, so buried lines may be closer to the surface than you think.