BBC reported that the unusual phenomenon (which forced a New Zealand naval vessel to shift its course) is probably pumice ejected by an underground volcano called Monowai.
"As far ahead as I could observe was a raft of pumice moving up and down with the swell," Lieutenant Tim Oscar told Agence France Press. "The [top of the] rock looked to be sitting two feet above the surface of the waves and lit up a brilliant white color. It looked exactly like the edge of an ice shelf," the officer said.
He added it was the "weirdest thing" he had ever seen in his 18 years at sea.