Srimad Bhagavatam Canto 05, Chapter 01, Text 32

SB 5.1.32

jambu-plaksa-salmali-kusa-krauñca-saka-puskara-samjñas tesam parimanam purvasmat purvasmad uttara uttaro yatha-sankhyam dvi-guna-manena bahih samantata upaklptah.
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
The names of the islands are Jambu, Plaksa, Salmali, Kusa, Krauñca, Saka and Puskara. Each island is twice as large as the one preceding it, and each is surrounded by a liquid substance, beyond which is the next island.
 
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
The ocean in each planetary system has a different type of liquid. How they are situated is explained in the next verse.
Srimad Bhagavatam Canto 05, Chapter 01, Text 31
Srimad Bhagavatam Canto 05, Chapter 01, Text 33