Yes I have heard of it...most beaches in NZ are littered with pumice. It's pretty brittle, full of air, and is very light...which is why it floats.

I don't think it would neccessarily be a overly dangerous to a ship per se in terms of being able to crack a hull or anything, but could pose a problem if a vessel got beached, so to speak, on it given the size of this pumice shelf.