NMOS Switch, Drain Source Location
If you are designing circuits at the IC level then your MOSFETs will be four-terminal devices and there is no physical difference between the source/drain terminals.

Most importantly this means that the body of the transistor need not be connected to the source. In fact, the body of all NMOS transistors is usually connected to the lowest voltage in the device, often ground. In this case the "source" of an NMOS transistor will be the terminal with the lower voltage and the "drain" terminal will be the one with the higher voltage.