Cancer was once a great sailor.
his ship was destroyed by sea serpent. the monster attacked the sailors ripping to shreds. hearing their pleads Capricorn drove way the beast. but unfortunately only one sailor barley survived.
she did what she could to save him, she used the spirit of the Great Crab to create a new body..Capricorn nursed him back to health.
the whole ordeal had left him with memory of his past. under the care of Capricorn and adjusting to his new body he slowly began to believe that he is a guardian for all those who live in the sea. after seeing the surface dwellers treated the sea Cancer would attack and sink ships, leaving the sailors to the sharks.
Capricorn tries to explain that not all of the surface dwellers are evil and that most wish to protect the seas as well.
Still not trusting the surface he listens to Capricorn. he now only attack ships that pollute and attack the wise ones (whales)

With the pearls given by Capricorn, Cancer can control the tides. Rising them to floods polluting villages. he can cause the tide to reseed to make it easier for him to get to land and attack those who rob from the see.
His new body is next to impossible to penetrate and his claws are capable of crushing marble

sorry if the bio is a little long