This terminology is independent of the depth of the water body, i.e., the benthic zone is not determined by the empirical depth at which the bottom lies, but rather by the state of being the bottom-most region.
In shallow lakes and along coastlines, the benthic zone is highly interactive with the pelagic zone (the zone above the benthic zone), and there is sometimes only a few inches or feet of water between the surface and the bottom. In oceans, though, the difference really starts to come to the fore.
Since the benthic zone is the zone at the bottom, it starts right at the coastline, and carries on along the continental shelf into the deep ocean. The benthic zone in deep waters is equated with abyssal plains. Since the conditions in the benthic zone in shallow waters are very similar to those in the pelagic zone, the description in the following Buzzle write-up refers to the benthic zone in deeper waters.
Organisms living in the benthic zone, particularly on the surface of oceans, are known as benthos. These are divided into two categories: Epifauna, which live on the surface, and infauna, which burrow into it.