Final answer:
The flat part of the margin covered by shallow water is the continental shelf, a submerged extension of a continent's crust in shallow waters, generally with a depth of less than 180 m (600 ft).
Step-by-step explanation:
The relatively flat part of the margin covered by shallow water is known as the continental shelf. It is the submerged portion of a continent's crust that extends from the shoreline out to the shelf break, where there is a significant change in slope. The continental shelf is predominantly composed of sand and silt and is characterized by decreasing energy levels with distance from shore. Its average depth is generally less than 180 m (600 ft). During times known as lowstand, when sea levels are low, the continental shelf and carbonate platforms may be exposed, leading to the formation of extensive deltas and considerable sediment transportation into deep marine basins.
On the continental shelf, you may find bed forms such as ripples and sand ridges, which are created through the interaction of currents and waves. The ocean bottom on the shelf is seldom completely flat due to these undulating features.