Short answers, pulled from the story.
The White Sea is a southern inlet of the Barents Sea on the northwest coast of Russia. It is bounded by Karelia to the west, the Kola Peninsula to the north, and the Kanin Peninsula to the northeast, and falls entirely under Russian sovereignty as part of the country's internal waters.
Kandalaksha Gulf, in the western part of the White Sea, is the deepest section, reaching 340 metres (1,115 feet). By contrast, the gorlo strait connecting the White Sea to the Barents Sea is shallow, with maximum depths of about 40-50 metres.
The White Sea served as Russia's primary international trade route from the 15th century through the early 18th century. The port of Kholmogory, and later Arkhangelsk, handled exports of fur and fish and hosted the Muscovy Company after Queen Mary granted it a charter in 1555 to trade via the White Sea route.
Richard Chancellor commanded the English ship Edward Bonaventure, which arrived at Kholmogory on the White Sea in 1553 while searching for a northern route to India and China. He was escorted to Moscow to meet Ivan IV, and on his return in 1554 he brought a letter from the Tsar expressing a desire to trade with England, which led to the founding of the Muscovy Company.
Yes, the White Sea freezes from October-November through April-May each year. Ice is typically about 40 cm thick but can reach 150 cm in cold winters; however, 90% of the ice is floating and drifts continuously toward the Barents Sea, and icebreakers keep the sea navigable all year round.
The White Sea-Baltic Canal is a waterway that connects the White Sea to the Baltic Sea via Lake Onega. It links the White Sea to Saint Petersburg and, through the Volga-Baltic Waterway, to the Volga River and the Black, Caspian, and Azov seas.