In this Discovery video, which has hit the tubes recently, you'll see what is called an oarfish. Normally fish that look like this don't rise anywhere near the surface of the sea. They say in the video that this critter is 5 to 10 meters long. If that is true, then it may account for the old mariner tales of sea serpents that were like fish, but huge and sporting tendrils. Aside from the giant squid, oarfish fits the bill perfectly, especially if it's :
A huge oarfish was caught on camera in the Gulf of Mexico, giving scientists a rare glimpse of the bizarre fish in its native deep sea habitat. Researcher Mark Benfield describes the fish, a likely inspiration for the sea serpent myth.