In many cases you will see that a water heater has been elevated a few feet off the ground. This is most common when the tank is installed in a garage. It is usually elevated to prevent any spilled liquids such as gas or oil that could interfere with the unit.
Some plumbers will install a block below the tank in order to prevent direct contact with the concrete. This was something that was recommended at one time but no longer required as the tanks are designed to sit directly on the concrete. If a water heater is installed on the first floor or above, it should sit inside of a water heater pan that connects to a drainage line. This line is usually not something that should be tied into the plumbing system because any p-trap that you add will often dry up.