I have a species of creatures that live in packs off the coast of my fictional world's one super continent. Humans have lived there for millions of years. At one point these humans had achieved the level of technological domination that we have over Earth. They mercilessly exploited resources and mixed species that should never have even met on a regular basis.
They generally screwed many ecosystems up. At some point the planet was unable to support them any longer. With no more resources left to exploit, human society collapsed. Rather than banding together to exploit the stars, humanity fought itself, with powerful individuals scrambling to ensure their place when the chaos ended. Over time, with no oil, or coal left, electricity became a rare commodity reserved for the powerful. The population plummeted from 10 to 1 1/2 billion. Because of these events technological progress stagnated. Literacy dropped. The world was stuck at mostly pre-industrial levels of technology.
Over millions of years species began to evolve to take advantage of the weaknesses the humans presented. Evolution caught up with humanity. Predators hunted humans for prey in many regions.
I want a creature in the oceans to evolve to eat voyaging humans. What characteristics and features does this creature need to survive and thrive? It will be very large, but not as large as a Spanish galleon. It will be a lone-hunter. I guess it would also have to be able to go for an extended time without food. It cannot go on land. Humans have to be its primary food source.
If this is too unrealistic, please tell me and tell me how I can improve my world. I have also tagged reality-check in this event.