Ser Corlys Velaryon was born in 280 AC to Lady Shaera Velaryon and her husband, Ser Daeron. A striking lad since the day he was born, many speculate that the true father of Ser Corlys was Prince Rhaegar.* Whether Ser Daeron knew of his 'son's' true heritage is unknown, but it is said he was fond of the boy, training him personally, and much like his mother and his namesake, Corlys took to the water with ease. 

Growing up, Corlys spent most of his time with his cousins, Monford Velaryon and Aurane Waters, the three becoming almost inseparable for a time. While he lacked the patience for more scholarly pursuits, it is said that the young Velaryon was adept with sword and bow. When he wasn't sailing, he could often be found training in the courtyard (or in seedy, dockside brothels.)

In 298, Corlys accompanied his mother Lady Shaera across the narrow sea to Essos with his sisters, Daella and Jacaera. There he pledged his sword to Viserys Targaryen (and later his sister, Daenaerys) with his mother, finding particular admiration of his fellow Westerosi, Jorah Mormont. When Corlys's father perished on the journey to Qarth, the young knight turned to Jorah for most of his advice. 

Following the death of his cousin at the Battle of the Blackwater, Corlys's mother returns to Westeros with his sister Daella, leaving Corlys and Jacaera behind in Essos with Queen Daenaerys and a quarter of his mother's personal fleet. 

*to be plotted with targaryen muses if alternative is preferred.