Crypt069A Sea Serpent

Sea Serpents have been reported by sailors for centuries. Though many were whales or drunken hallucinations, there are a number of sightings that consistantly described the same animal- these sightings persist into modern times.
The typical Sea Serpent is massive in length, anywhere from 60-100 ft, but with a thin, elongated build. They are described as scaly with a snake-like head: mariners of old frequently referred to them as giant blacksnakes, and they are sometimes said to vaguely resemble anacondas.
They are often described as being devoid of fins- though when fins are included, they are consistantly described as being squarish in shape, usually being taller in the front than in the back. Translucent pectoral fins, located just behind the head, are also commonly reported. They swim side-to-side, like a reptile or fish, rather than undulating vertically like an aquatic mammal.
Sea Serpents are constantly said to be either black or very dark green, with a white underbelly. A tan or cream colored horizontal stripe is also reported, separating the darker and lighter parts of the body and running down the entire length of the animal.

After reading up on these creatures, I've decided that the culprit may be some kind of gigantic fish. There are many long-bodied fish that look like reptiles (eels being the most obvious example), and since they also locomote the same way, it wouldn't be impossible for people to get them mixed up with true reptiles. I looked up serpentine fish and found one called the Rope Fish. They're small, freshwater fish that only live in tropical waters, so they're not much of a candidate (besides the size different, Sea Serpents are also saltwater creatures are are seen pretty regularly in frigid waters). Still, they look a LOT like the average Sea Serpent, so I based mine off them. If Sea Serpents are fish, I have no idea what specific family they might hail from.