SB 5.23.4
kecanaitaj jyotir-anikam sisumara-samsthanena bhagavato vasudevasya yoga-dharanayam anuvarnayanti.
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
This great machine, consisting of the stars and planets, resembles the form of a sisumara [dolphin] in the water. It is sometimes considered an incarnation of Krsna, Vasudeva. Great yogis meditate upon Vasudeva in this form because it is actually visible.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Transcendentalists such as yogis whose minds cannot accommodate the form of the Lord prefer to visualize something very great, such as the virat-purusa. Therefore some yogis contemplate this imaginary sisumara to be swimming in the sky the way a dolphin swims in water. They meditate upon it as the virat-rupa, the gigantic form of the Supreme Personality of Godhead.