The Shark Incarnation Ceremony.
"My son".
This one magical word was what performed the magic.
Before meeting his dad, Sebastian planned a lot on how he would comport himself, he planned how he would talk and how he would behave, including how he would react to his words but his brain short-circuited on finally meeting him.
This feeling was not entirely due to the intimidating stature of his father but was more because of the bond that resonated between the both of them through their blood.
Inherently, Sebastian's soul was an earthling but his body in Oceania was a shark and its bloodline came from this Megalodon.
This was why the bond between the both of them was so strong.
Immediately after his father called him son, this magical word evaporated all the misgivings in Sebastian's mind. His intuition already proved to be correct in most circumstances, and he believed that it was right this time around also.
This massive Megalodon was his father.