This isn't a sea monster walking the bottom of the ocean- it's a school of juvenile striped eel catfish working in unison to look menacing enough to ward off predators. This was filmed off the coast of Bali. The Kid Should See This has more:
Try rewatching the video, picking one fish and following it the entire time. Then pick another fish and watch the video again. The juvenile striped eel catfish seem to cycle through positions within the school as the entire swarm moves forward.
I was captivated by the ones at the bottom, that seem like pixels in an animation of a creature walking on thick legs. (via Kottke)