Mako became a diver because he wanted to do something interesting, the ocean was calling to him. On his first solo dive Mako found himself in the middle of an ancient temple, surrounded by glittering jewels and riches.
The temple around him rumbled and the water turned a deep shade of red. A tendril of murky black energy slunk out from a statue of a shark at the far end of the room and wrapped itself around Mako, diving straight through his skin and into his heart. The energy belonged to the spirit of a long lost water God, awoken by Mako's presence. It nestled inside of Mako's body and began to change him, transforming him into a shark.
The spirit explained that Mako had been chosen to fulfill a great destiny, he would become the guardian of Anuran's shores, bringing the wrath of the gods down on anyone who damaged them. the oceans had a new protector, at least when he's not out partying on the beach, anyway