Sea of Madness is a track from the Somewhere in Time album, that is a metaphor for the current state of mankind. It could be linked to the Bosch 15th Century painting called the Ship of Fools, an allegory where artists were representing civilisation as a ship with a crew of fools set adrift on a sea of madness.
Of course more locally, the sea here hides a different sort of madness that geology has unleashed on Christchurch in recent years, with faultlines just off the coast causing mayhem and madness of their own.
Meanwhile, there are plenty of caches in the area, a veritable sea of them :)