The Giant Pacific octopus can grow to very large sizes in very short amounts of time. After they hatch they have the ability to turn around 60% of what they eat directly into body mass, making them one of the most efficient carnivores on the planet. This allows them to double their size every 100 days.
However, these intelligent shape-shifters live fast and die young. Their lifespan only extends about 3-4 years.
Additionally, the suckers on some of the larger specimens of giant Pacific octopi can support up to 35 pounds of weight. Multiply this by 200 suckers per arm and again by 8 arms and you have a creature that can muscle multi-ton forces; more than a match for any careless diver.