Thinking about "sea serpents" today. Bernard Heuvelmans may have come up with 358 reports, but how many would you be willing to introduce in, say, a court trial, with serious stakes involved (however that might happen)?
We all agree the gold standard for sea serpent reports is the Nicoll/Meade-Waldo sighting of 1905, but what else would you consider grade-A eyewitness evidence? The Daedalus, given the watch officer's sketch that makes the animal much smaller and further from the ship than the famous Illustrated London News picture, might have been a squid...
My top additional sightings? HMS Fly's encounter is pretty inexplicable, as is the SS Umfuli's. The French sightings off SE Asia around 1898 seem compelling, but all we have is Heuvelmans' description of them - to my knowledge, the original naval reports have never been translated into English. HMS Hilary (1917) and Stimson Beach (1983) qualify as unexplained. Mackintosh Bell's sighting of what seems like a giant long-necked seal is another one I'd consider positive evidence. Finally, the New England Sea Serpent of 1817-on is impressive by the sheer mass of the evidence: even the very cautious Richard Ellis thinks something peculiar was going on.
I believe there is at least one real creature behind "sea serpent" sightings, but I have a hard time filling out a Top 10 list of what I think are unexplainable sightings. None of the strandings, carcasses, or photos hold up as something not otherwise explainable. Carcasses that were not thoroughly examined don't count, only ones that were see by someone with scientific training and confirmed as an unknown species... and there have been no such incidents.
Do we have other nominations to go in a Top 10?