All the islands (the seven islands in the planetary system known as Bhu-mandala) are completely surrounded by these oceans (consisting of salt water, sugarcane juice, liquor, clarified butter, milk, emulsified yogurt, and sweet drinking water)
SB Canto 5
The seven oceans respectively contain salt water, sugarcane juice, liquor, clarified butter, milk, emulsified yogurt, and sweet drinking water. All the islands are completely surrounded by these oceans, and each ocean is equal in breadth to the island it surrounds. Mahārāja Priyavrata, the husband of Queen Barhiṣmatī, gave sovereignty over these islands to his respective sons, namely Āgnīdhra, Idhmajihva, Yajñabāhu, Hiraṇyaretā, Ghṛtapṛṣṭha, Medhātithi and Vītihotra. Thus they all became kings by the order of their father.
It is to be understood that all the dvīpas, or islands, are surrounded by different types of oceans, and it is said herein that the breadth of each ocean is the same as that of the island it surrounds. The length of the oceans, however, cannot equal the length of the islands. According to Vīrarāghava Ācārya, the breadth of the first island is 100,000 yojanas. One yojana equals eight miles, and therefore the breadth of the first island is calculated to be 800,000 miles. The water surrounding it must have the same breadth, but its length must be different.