Use a Resource Bare Island
Humans have a bad habit of adapting to dangerous situations which makes it difficult get a stable supply of them. The best recourse then is to put them on an island so that they have nowhere to run too. Ideally you should clear the island of all trees and pointy rocks first to make sure they can not make any boats to run away with or weapons to fight back.
Once you take away the resources human can use to make all their nasty inventions with, they are basically just caged monkeys. At this point of course, they will not be able to feed themselves very well; so, you'll need bring them regular feedings, but making them rely on you to feed them will also force them to the shores making snatching them up much easier.
As for what you feed the humans, most seafood is pretty lean compared to terrestrial food sources; so, you can't just feed them a bunch of seaweed and fish and expect them to get fat. Instead you will probably need to prepare your human food with added fats and/or sugars. Added sugar is hard to process underwater; so, I would suggest "enriching" thier food with the blubber from whales, seals, or dolphins to make sure they put on enough calories to get the humans all fat and tasty themselves. Because mammals store fat in separate layers from thier muscles this makes properly butchered mammal meat a better source of empty calories than fish.
As for the Bounty looking for a credible source:
Your need a system that does two things very well. 1: It must detain people, and 2: it must make it easy for a water predator to catch land based prey. So, this question draws inspiration from two sources that are each individually REALLY good at that.
When it comes to detaining human beings, the best experts in the world at that are the people who design and run maximum security prisons. They regulate the hell out of what the prisoners are allowed to own because they often use relatively mundane items to fashion tools and weapons. By stripping the island of resources, you are replicating this precaution. The second thing they do is create an exclusion zone around the prison to make sure there is no where to run to. While many prisons use deserts or other kinds of wilderness, the most effective exclusion zone is always water such as you see with Alcatraz prison; so, using an island is not just about what is convenient for your aquatic life forms, but what is hardest for your humans to get away from.
As for the water predator, I take a certain amount of inspiration from Nile crocodiles. Most aquatic animals don't have much luck hunting land based animals, but for crocodiles it is easy. In places where they live, land animals are 100% reliant on the water where the crocodiles live if they want to survive. By feeding the humans, you create that same absolute need for humans to go to the water where the predators are for survival. If you can bring them to the water, then like the crocodile, you don't need to leave the safety of your home turf advantage to pick off otherwise dangerous prey.