The average depth of the sea is about 1,000 meters (3,300 ft). The northern and eastern parts are shallower than 180 meters (600 ft) due to the silt deposited by the Irrawaddy River. This major river flows into the sea from the north through Burma. The western and central areas are 900–3,000 meters deep (3,000–10,000 ft). Less than 5% of the sea is deeper than 3,000 meters (10,000 ft). In a system of submarine valleys east of the Andaman-Nicobar Ridge, the depth is more than 4,000 meters (13,200 ft). The sea floor is covered with pebbles, gravel and sand.
The coastal areas of the Andaman Sea have mangrove forests and seagrass meadows. The mangrove roots trap soil and sediment. They provide shelter and are a nursery for fish and small aquatic organisms. Their body protects the shore from the wind and waves. Their detritus are a part of the aquatic food chain.
Other important sources of nutrients in the Andaman Sea are seagrass and the mud bottoms of lagoons and coastal areas. Many aquatic species move from and to seagrass either daily or at certain stages of their life cycle.