MUSCAT, Oman — Passengers on a luxury cruise liner attacked by pirates in the dangerous waters between Yemen and Somalia said Wednesday they were surprised by the assailants' boldness and described hearing the "Pop! Pop! Pop!" of the pirates' rifles firing at the ship.

Sunday's attack on the nearly 600-foot long American-operated M/S Nautica in the Gulf of Aden was the latest evidence that pirates have grown more aggressive, viewing almost any ship on...