The sea territories of Hai Phong are part of north-eastern water area of Gulf of Tonkin. The sea bed compositions and oceanographic characteristics of Hai Phong are closely connected to the common characteristics of Gulf of Tonkin and Eastern Sea.
The depth of Hai Phong sea is not large. A depth curve of 2m goes around Do Son Cape and then descends down to 5m far from the coast. At the sea bed where the rivers flow into the sea, the depth is greater due to erosion caused by currents. Further offshore, the level of sea bed descends to the depth of the Gulf of Tonkin of about 30 – 40m. The surface of Hai Phong sea bed is formed by fine components with many deep underwater canals which were used to be river beds and are currently used as daily navigation channels for vessels.
The length of sea coast of Hai Phong is 125 km including the length of coast surrounding the offshore islands. The coast has a form of a concave curve as the common sea side of the Gulf of Tonkin. It is low and quite flat with the structure created mainly by muddy sand coming from the 5 major estuaries. At the center of the coast, Do Son cape stretches out into the sea like a peninsula. This cape is also the end of the range of mountains of Devonian sandstone running from the mainland. The highest point is 125m, and it stretched out 5km into the sea in the direction of North West – South East. This advantage of natural structure has given Do Son a status of an important strategic location in the sea and also a famous spot for its beauty. At the foot of the sandstone hills there are beaches making this place a romantic resort and a valuable convalescent area. In the offshore territory of Hai Phong there are many islands which are distributed over the sea with Cat Ba Island as the largest and Bach Long Vi Island as the furthest.
Sea, coast and islands have created special natural landscapes to this coastal city. It is also a special potential advantage of the economy of Hai Phong.