Size: Variable. Reports can be as small as ten feet, to hundreds of feet long.
Appearance: Long, serpentine bodies with dragon or horse-like heads.
Threat: Medium. Many sea serpents could crush ships just by sheer size, and huge jaws lined with sharp teeth means predators that would likely not pass up the chance to devour some hapless sailors. There are almost no actual accounts of such creatures actually attacking ships or people though. It could simply be because there are no survivors.
Reports of sea serpents have dwindled since we've filled in most corners of the map. That doesn't mean there aren't any though. The news usually has three to four stories a year about unusual creatures washing onshore or a strange shape seen in the water from a ship.
It's been said we know more about the surface of the moon than we do about the depths of our oceans. Even the giant squid was just a myth until several years ago, and there's hints of even bigger versions down there. Squid up to 120 feet long. So it's not at all unbelievable that a 60 - 75 foot serpent could be gliding through the water out there, preying on sharks, or even small whales.
Many things are probably just being misidentified though. A giant squid close to the surface could
easily give the impression of a single, or even several long, snake-like creatures. Especially if one tentacle lifted up out of the water, it would certainly look like a long neck with a small head at the end. A lot of times, storms or catastrophic events can drudge up deep sea creatures and toss them on the beach. To an untrained eye, a 20-foot oarfish is easily a sea monster. Not to mention the rotting carcasses of whiles that wash up every so often, giving the appearance of something entirely different.
One of the things that's always struck me is how much many reports sound like creatures we know used to exist. Even reports that date back to before we knew of them. Plesiosaurs for one example, the sea-going reptiles that swam alongside the dinosaurs and that are commonly pointed to in claims of lake monsters. Long neck, dragon-like head, humps. Ocean species may have developed leaner, more snake-like bodies as well, especially in tropical waters.
Plesiosaurs are relatively small fry though. The real sea monsters would be Liopleurodon or one of the Mosasaurs.
Sea monsters are a staple in movies, such as Deep Rising, or Predator X, and Books abound as well. Tim Curran's Leviathan, Max Hawthorne's Kronos Rising, and others are relatively easy to find. Once you expand into sharks, there are literally hundreds of choices in movies and reading to keep a sea-fearing mind busy.
Perfect entertainment if you're planning on taking a cruise.