Your species won't be able to function as it is currently written. Humans in general are a terrible prey choice for a predator that is specialized on hunting a single type of prey. Humans are slow-breeding and rely on extremely low infant mortality rates in order to maintain a viable population. The only reason humans have an overpopulation problem now is that we don't have any reliable means of population control, and so even our painfully slow reproductive rate allows us to multiply like rabbits without any external checks on our numbers. Add in an actual predator that could reliably prey on us without being wiped out and our numbers would rapidly dwindle. Any predator that preys solely on humans would have to exist at extremely low population densities in order to function or else risk exterminating its food supply.
On top of this, humans have a tendency to deal with predators through campaigns of proactive violence. One of the first things large human populations tend to do when they establish themselves in an area is systematically wipe out all of the megafaunal predators. This is mostly because humans are very bad at defending themselves if caught by surprise, and rely on campaigns of proactive violence to maintain a buffer zone between the predators and their settlements where they are most vulnerable or have vulnerable offspring. A species which preys only on humans would be at the top of the extermination list and would be killed whenever possible. Humans have wiped out entire species just because we've thought they might be a threat to our farm animals, how do you think we would react if an animal was a threat to us directly?
On top of all these issues is the fact that your species is specialized to feed on an animal which doesn't even normally occur in its environment. On land you could at least make the argument that a predator could seek out and hunt down humans, but the only humans travelling on the water are going to be either sailors, fishermen, or swimmers, a mere fraction of the human population. There simply wouldn't be enough humans travelling on the water to be a viable food source. This is even before you consider humans just plain avoiding water travel because they know its dangerous. If all of the humans in an area avoided boat travel for a couple of years, your species would starve to extinction.
An animal the size of a Spanish galleon also wouldn't be able to survive on a diet of rare humans alone. A Spanish galleon is about 30-50 m long, and the largest carnivorous whale, a sperm whale, is only 20 m at maximum. Assuming similar body proportions to a sperm whale but scaled up to the size of a Spanish galleon and a cold-blooded metabolism of a saltwater crocodile, your predator would weigh approximately 1600 tons (1.45 million kg) and require 525000 kilocalories per day (noting this is a very approximate estimate because estimation equations get wonky when you extrapolate them to kaiju size). Scaling that by the number of calories in the human body, which is about 125,000 kcal, and your predator would have to eat two and a half humans every day just to sustain its energy budget. This is assuming your predator does not waste any energy hunting food. The amount of energy required per kg would be low, but the total energy needs would still be enormous. It wouldn't be enough for a species to survive.
On top of that, you have growth to consider. Your predator has to actually reach 30 m (and now that I think about it, it's not clear what they would eat when they are young and cannot sink ships and are possibly too small to even eat infants). Growth is a trade-off between how much metabolic energy you burn and how fast you grow. Crocodiles, for example, grow incredibly slowly and can take in excess of 50 years to reach monstrous sizes. Giant extinct crocodiles like Deinosuchus and Purussaurus are thought to have lived over a century. By contrast, whales and dinosaurs reach maximum size in less than a decade but have very high metabolisms. Given your species incredibly meager energy budget, it would have to have an incredibly slow growth rate to survive on its meager diet. This would make your species very vulnerable to being hunted to extinction by humans. Slow-breeding, long-lived species generally don't do well when humans are around.
A much better option would be something like crocodiles, sharks, or goonch catfish. Normally feeding on thinks that are abundant in an aquatic ecosystem like fish and turtles, but who have become accustomed to viewing humans as food due to human behavior (like burying their dead on funeral pyres or dumping waste offal from slaughterhouses into the water). This would cause your predator to associate humans with food and therefore seek out humans when they are hungry. It would also cause them to disproportionately associate themselves with human settlements, as the humans are unintentionally encouraging them to stick around by providing them with free food. All the predator needs to do is be smart enough to avoid being hunted.
