Well, I suppose that one could be a coincidence, but that still doesn't explain why it took Manaka five years to start dying if she was to die as a sacrifice to the sea god. Unless you think Manaka's dying wish was to show the undersea town to Miuna, and Uroko needed to wait five years for Miuna to take a dip. Maybe he got tired of waiting and woke up Hikari to give her a push. Then he kicked Kaname out of the sea to make sure they would have the group born bravery to go diving.