Srimad-Bhagavatam: Canto 5 - Chapter 1 - Verse 4
केचनैतज्ज्योतिरनीकं शिशुमारसंस्थानेन भगवतो वासुदेवस्य योगधारणायामनुवर्णयन्ति ॥ ४ ॥
kecanaitaj jyotir-anīkaṁ śiśumāra-saṁsthānena bhagavato vāsudevasya yoga-dhāraṇāyām anuvarṇayanti.
This great machine, consisting of the stars and planets, resembles the form of a śiśumāra [dolphin] in the water. It is sometimes considered an incarnation of Kṛṣṇa, Vāsudeva. Great yogīs meditate upon Vāsudeva in this form because it is actually visible.
Transcendentalists such as yogīs whose minds cannot accommodate the form of the Lord prefer to visualize something very great, such as the virāṭ-puruṣa. Therefore some yogīs contemplate this imaginary śiśumāra to be swimming in the sky the way a dolphin swims in water. They meditate upon it as the virāṭ-rūpa, the gigantic form of the Supreme Personality of Godhead.