On the north side of Cyana in Okasankar there are 2 boatmen, the one on the left Zidar will take you to Zorkatraz for 150 zorkins. You have to follow the boat by swimming, and once arrived you need to swim under a bridge and into the centre of Zorkatraz onto a platform with stairs in the centre of the water before you can start shooting the soldiers. The captain of this group also drops a key to unlock the prisoner if you did not already have the Zorkatraz key.