SB 9.6.39-40

yamunāntar-jale magnas
 tapyamānaḥ paraṁ tapaḥ
nirvṛtiṁ mīna-rājasya
 dṛṣṭvā maithuna-dharmiṇaḥ
jāta-spṛho nṛpaṁ vipraḥ
 kanyām ekām ayācata
so ’py āha gṛhyatāṁ brahman
 kāmaṁ kanyā svayaṁvare
Synonyms: 
yamunā-antaḥ-jale — in the deep water of the river Yamunā; magnaḥ — merged completely; tapyamānaḥ — executing austerities; param — uncommon; tapaḥ — austerity; nirvṛtim — pleasure; mīna-rājasya — of a big fish; dṛṣṭvā — seeing; maithuna-dharmiṇaḥ — engaged in sexual affairs; jāta-spṛhaḥ — became sexually inclined; nṛpam — unto the King (Māndhātā); vipraḥ — the brāhmaṇa (Saubhari Ṛṣi); kanyām ekām — one daughter; ayācata — begged for; saḥ — he, the King; api — also; āha — said; gṛhyatām — you can take; brahman — O brāhmaṇa; kāmam — as she desires; kanyā — daughter; svayaṁvare — a personal selection.
Translation: 
Saubhari Ṛṣi was engaged in austerity, deep in the water of the river Yamunā, when he saw a pair of fish engaged in sexual affairs. Thus he perceived the pleasure of sex life, and induced by this desire he went to King Māndhātā and begged for one of the King’s daughters. In response to this request, the King said, “O brāhmaṇa, any of my daughters may accept any husband according to her personal selection.”
Purport: 

This is the beginning of the story of Saubhari Ṛṣi. According to Viśvanātha Cakravartī Ṭhākura, Māndhātā was the king of Mathurā, and Saubhari Ṛṣi was engaged in austerity while submerged deep within the river Yamunā. When the ṛṣi felt sexual desire, he emerged from the water and went to King Māndhātā to ask that one of the King’s daughters become his wife.