The waves for the final ships appears to be bugged. After finishing the veteran mob of the gladiator ship Elli says her speech, Jeyne drops the gangplank and the two seem to freeze, whilst everyone else follows to the next boat where nothing spawns. I’ve tried this twice now and the same result occured each time.
Did you talk to them while they were building the path to the next ship? I’ll try to reproduce this (I’ve completed this story step several times) but I’ve seen this pop up occasionally for events/story steps. (Sometimes talking to the NPC will make them abort what they were doing.)