Speak with Admiral Marc Grenich on the Qeynos Harbor docks and tell him you're willing to take up the cause to defend Qeynos.
Ring the mariner's bell on the docks that would normally take you outside Qeynos and choose to enter the Kunark Expedition. You will arrive aboard the QSS Valiance.
Speak with Captain Pegg Bristol to trigger the attacking ship. Iksar sailors will bring their ship along side the Valiance and board the ship. You cannot board the attacking ship. The attacking sailors will board the Valiance and will begin killing ship crew members but remain unattackable to you. Once the sailors become attackable, they will attempt to kill you.