Oarfish.

From the website:

The serpent-like animal was found six metres offshore, bringing to at least six the number of oarfish that have washed up on the West Australian coast in recent months. Prefering to live in the depths of the ocean they have only been known to come to the surface when sick or dying and have rarely been seen alive.

Living in the world’s warmer oceans, it feeds on plankton and is harmless to humans. The longest bony fish in the sea, it grows up to nine metres long with a bright red crest that runs the entire length of its body.

It is probably the creature that sparked "sea serpent" legends following sightings by ancient mariners.

Link (via Underwater Times)