Sea Creatures, Cryptids, and Seals

One of my favorite topics in cryptozoology is "sea monsters" or "sea serpents."  Now, no one believes in giant marine snakes, and very few biological scientists believe in possible sea monsters at all, but there are a lot of people like myself who can't let go of the idea that something peculiar still lurks in the stories and legends of giant, sometimes terrifying creatures from the deep. Some reports are traceable to real creatures, like these oarfish.  And the stories have provided inspiration  for toys, hoaxes - sophisticated or silly - knick-knacks, books, movies, and so on.
The question is - are all such stories dismiss-able?
There's been a spate of recent blog/internet posts on the topic, and some make for intriguing reading.
A common speculation among cryptozoologists is that there's an unknown pinniped (seal) with a long neck.  Seals have surprisingly long necks, though it's often not obvious because there is a lot of fat under the neck fur.  The leopard seal, which is genuinely scary (and it should be, because it's definitely killed humans) looks like a long-necked reptile from some angles.  The species' length is often cited as up to 10 feet, but men from the famed (doomed) exploration ship Endurance killed one as it was attacking and measured it at 12 feet.
I learned that from Karl Shuker's blog. Dr. Shuker (he's one of two Ph.D. scientists in the world who write on the "pro" side of marine cryptids: Darren Naish, much more cautious but still intrigued, is the other), published a two-part blog on the long-necked seal idea.  (Part One, Part Two) He's pretty thorough.  He ends up being very cautious: he notes there's fossil evidence for seals with slightly longer necks than modern ones, but nothing that could be taken for a swan-necked plesiosaur-like animal.  He admits one case really stumps him: a closeup sighting by British lawyer Mackintosh Bell and a cod-fishing crew in 1919.  Bell described his animal so thoroughly at close range that there are only two possibilities: Bell saw a long-necked seal, or the whole account is a lie. (It does not appear the fisherman friends who accompanied him ever set down accounts, so this case does depend on Bell's word, but he stood by it in correspondence with oddities investigator Rupert J. Gould.
Sea monsters are not popular these days: indeed, for most scientists, they never were. (Sir Richard Owen was an early and vociferous critic. I must note his treatment of the Daedalus crew in 1848 was unfair, despite the fact the latter probably saw a giant squid.)  
But the grandfather of modern sea serpent stories, the Gloucester beast, arguably remains not quite explained.  See Craig Woolheater's article for a good review.  The episode started in 1638 and peaked in 1817. It's always been puzzling: it may may forever be puzzling.  Even a cautious authority like Richard Ellis thought something strange had happened.  The oft-invoked (and often true) explanation of "contagion" for "flaps" of odd occurrences seems inadequate: People were reporting big animals of a fairly consistent description that puzzled men who'd spent many years at sea.
In the 21st century, years can pass without a sea serpent report, near Gloucester or anywhere else.  Is whatever animal might be at the basis of sea serpent stories rare, extinct - or was it never real?
I'd like to think it's just rare.  I could be wrong. But I hope I'm not.