Marine deposits are a variation on alluvial deposits.
They result from the wave action of the ocean, which has concentrated diamonds at the base of the surf zone. Waves arriving at an angle to the coast tend to push the diamonds along the coast, causing the diamonds to stream out from where rivers deposited them at the coastline. There are 3 types of marine mining operations. In one, sand is moved from 10 meters below sea level to as far inland as the sea may have risen, in order to reveal the concentrations on the bedrock. In another, divers and boats work in the surf zone to perhaps 20 meters of water and use suction pipes to remove gravel and diamonds from the ocean floor.