There is no "sea serpent" if one means an actual serpent. Whether there is some form of large elongated marine mammal, be it eel or seal or whatever, yet to be classified is a much more interesting topic.
Cadborosaurus, the name given to a creature oft reported from the Pacific Northwest off Canada and the US, has popped up again with the claim that a school or herd of Caddys was filmed last year in an estuary off the Gulf of Alaska. A group of marine creatures, certainly, was filmed, but are they clearly outside the realm of known species? Longtime cryptozoological researcher John Kirk has seen the film and thinks we have a breakthrough here. Unfortunately, the rest of us must wait until a still-untitled cable TV show in September to see it for ourselves.