Do you want to be up on the poop deck or down on the very bottom? The choice is yours. Crustaceans are "bottom dwellers", feeding on waste from other creatures. They actually have to "stir things up" with their appendages, in order to feed.
The poop deck is a place that is elevated for vision. It is the place of observation and navigation. Poop is from a Latin word puppis that simply means elevated. On sailing ships, the helmsman, who steers the ship is on the poop deck.