SB 3.29.42

sravanti sarito bhītā
 notsarpaty udadhir yataḥ
agnir indhe sa-giribhir
 bhūr na majjati yad-bhayāt
Synonyms: 
sravanti — flow; saritaḥ — rivers; bhītāḥ — fearful; na — not; utsarpati — overflows; uda-dhiḥ — the ocean; yataḥ — because of whom; agniḥ — fire; indhe — burns; sa-giribhiḥ — with its mountains; bhūḥ — the earth; na — not; majjati — sinks; yat — of whom; bhayāt — out of fear.
Translation: 
Out of fear of the Supreme Personality of Godhead the rivers flow, and the ocean never overflows. Out of fear of Him only does fire burn and does the earth, with its mountains, not sink in the water of the universe.
Purport: 

We can understand from the Vedic literature that this universe is half filled with water, on which Garbhodakaśāyī Viṣṇu is lying. From His abdomen a lotus flower has grown, and within the stem of that lotus flower all the different planets exist. The material scientist explains that all these different planets are floating because of the law of gravity or some other law; but the actual lawmaker is the Supreme Personality of Godhead. When we speak of law, we must understand that there must be a lawmaker. The material scientists can discover laws of nature, but they are unable to recognize the lawmaker. From Śrīmad-Bhāgavatam and Bhagavad-gītā we can know who the lawmaker is: the lawmaker is the Supreme Personality of Godhead.

It is said here that the planets do not sink. Since they are floating under the order or energy of the Supreme Godhead, they do not fall down into the water which covers half the universe. All the planets are heavy, with their various mountains, seas, oceans, cities, palaces and buildings, and yet they are floating. It is understood from this passage that all the other planets that are floating in the air have oceans and mountains similar to those on this planet.