~||Megaladon||~

Size and Health: If food was plentiful, it would grow to its maximum size, possibly exceeding normal estimates, and would be incredibly healthy and strong.
Diet: He would eat everything he was given, without showing aggression, but with characteristic shark-like grace. His diet would consist of huge portions of fish and seals (if given).
Behavior: He would become extremely lazy and spoiled, but would still retain his power. His interactions with humans would be as providers of food and entertainment, not as prey, but an attack on a ship would still be disastrous.
Danger: He would remain dangerous due to his size, strength, and instincts. Even in a pool, he could accidentally destroy something or someone. However, hunger wouldn't drive him to attack, but rather curiosity if he were "petted."
Housing conditions: He would need an ocean pool the size of a small bay to swim comfortably. He would have to be fed tons of fish daily.
World and people:
Scientific miracle: It would be a major scientific object, generating billions of dollars and changing our understanding of biology.
Tourism: A huge attraction where people would flock from all over the world to see a living giant.
Risk: There would always be a risk of escape or unforeseen consequences, as this is a prehistoric predator the size of a bus, albeit a well-fed one.
All in all, it would be a fascinating, yet still dangerous, reality where a man tries to tame an ancient monster, which is essentially just a very large, well-fed, and slightly spoiled shark.