The Baltic Sea is small and surrounded by several states

The narrow straits of Denmark are vital for the Baltic Sea, as they are the only connection between the Baltic Sea and the oceans. The outer boundary of the Baltic Sea is defined by the border between the Danish straits and the Kattegat.

The Danish straits refer to two adjacent waterways that connect the Baltic Sea to the North Sea. One of these narrow waterways is the Sound (Öresund), which runs between the southern coast of Sweden and the Danish island of Zealand. The other is the Belt Sea, located between the island of Zealand and the Jutland Peninsula. It is divided into two straits: the Great Belt and the Little Belt.

All these straits are shallow, which limits the water exchange between the Baltic Sea and the North Sea. The shallowest is the Öresund, with a minimum depth of only 7–8 meters. Another shallow point is in the Belt Sea, south of the Danish straits. This Darss Sill rises to 18 meters above sea level. It determines what types of vessels – how deep-draft ships – can enter the Baltic Sea.

When a ship entering the Baltic Sea passes through the Danish straits, it first encounters the Arkona Basin, located between Sweden and Germany. From there, the ship continues its journey to the Bornholm Basin, which spreads out east of the island of Bornholm.

Further east or north, the Gotland Basin is encountered. It is divided into three parts: the eastern, western, and northern Gotland Basins. On the side of the eastern basin, there are two large bays: the Bay of Gdańsk in the south and the Gulf of Riga further north.

The extensive area covered by the Arkona, Bornholm, and Gotland Basin is often referred to as the Baltic Proper or alternatively the main basin of the Baltic Sea. Here, the sea depth generally varies between 50 and 100 meters, but in some places, it exceeds 200 meters.

The shallowest areas of the proper Baltic Sea are in the south. The deepest point, however, is located in the northern part of the western Gotland Basin. This Landsort Deep reaches 459 meters, making it the deepest point in the entire Baltic Sea.

The sea areas surrounding Finland are bordered to the south by the northern Gotland Basin. The Gulf of Finland is a direct extension of the Gotland Basin, as there are no bottom ridges or sills at the mouth of the gulf to restrict water circulation. The average depth of the Gulf of Finland is 37 meters. The deepest point, at 123 meters, is located off the northwest coast of Estonia.

The northernmost basin of the Baltic Sea is the Gulf of Bothnia, which is quite separate from the rest of the Baltic Sea. The Gulf of Bothnia is divided into the Archipelago Sea, the Åland Sea, the Bothnian Sea, and the Bay of Bothnia. The deepest point in the Bothnian Sea is 293 meters, and in the Bay of Bothnia, it is 146 meters. These depths are closer to Sweden than to Finland.

Between the Bothnian Sea and the Bay of Bothnia lies the narrow and shallow Kvarken, with a sill depth of only 25 meters at its deepest, which restricts water flow. The fragmented Archipelago Sea also slows the movement of seawater from the main Baltic Sea to the Bothnian Sea.

The Åland Sea is located in a fault zone, and its bottom features are steep. In the strait between Åland and Sweden, near the Märket lighthouse, the 300-meter mark is broken. The exact depth is 301 meters. This is the only area in the Baltic Sea where such great depths are reached close to land.