Length: 75 - 200 feet long, from tentacle to tentacle.
Appearance: The same as any other octopus, except for its monstrous size. The shape and colors of Octopuses are highly variable though, they're known to be able to mimic many other ocean species, including eels, rays, and fish. They can even change their shapes to resemble rocks and coral.
Threat: moderate. Octopuses are predators, with a hard, sharp beak that is easily capable of cracking open shellfish or piercing flesh. They are also known to eat just about anything they can catch, which includes sharks and even birds. An octopus of significant size would have no problem grabbing and devouring a human being.
While occasional Globsters, like the picture above, are attributed as possibly being giant Octopuses, there is little to actually connect them to such animals. Most such unidentified masses are either found to be decomposing whale parts, or are washed up and then washed back out before they can be tested upon.
Where are the dark places these creatures would hide though? The answer for the Lusca is the myriad of underground rivers and waterways which run underneath central america, connecting many of the famous Cenotes, or blue holes.
These dark tunnels stretch for miles, connecting the inland cenotes with underwater caves out in the gulf of Mexico and the Caribbean. Many are huge caves as well, easily large enough for a giant, and malleable creature to slip in and out of. The cave walls would also give an octopus ample opportunity to blend into the rock, where only a truly dedicated eye would be able to pick them out.
Giant octopuses (That is the correct term, feel free to look it up.) have been fairly popular in movies. Headlining in such titles as the classic It Came from Beneath the Sea, Tentacles, and the aptly titled, Octopus. It's also had cameo's in such movies as King Kong, and some versions of 20,000 Leagues Under the Sea. So there's lots of options out there to see them in action.
We're not even going to touch this though.