One of Ivul Hleryn's rivals owns a trade ship that he wishes attack. Corter tells you to do the deed, and will provide you with 2x Potions of Water Walking and a Cheap Rising Force potion to assist you with boarding the vessel.
The ship itself is named the Dread Minotaur and can be found sailing the waters of Lake Coronati. You only need to kill the three crew above decks - you do not need to kill the slaves below. Your journal will update once all necessary individuals are dead. At this point, you can return to Corter - his disposition will increase by slightly more if you also freed the slaves, but his reward remains the same.
The following Journal ID and Index codes can be used with the Journalconsole command to manually update the quest to a certain point.
Assault on Lake Coronati (TR_m7_FG_HL_Q5)
Index
Finishes Quest
Journal Entry
10
Corter ordered me to attack a trade ship of Ivul Hleryn's enemies called 'Dread Minotaur' and kill everyone on the ship's crew. He told me the ship is currently anchored in the middle of Lake Coronati on its way to Narsis, east of Hlerynhul and southeast of Ald Iuval and Ald Marak.
50
Everyone on the ship's crew is dead.
100
Finishes quest
Corter paid me for the assault on the trade vessel.