"Despite centuries of alleged sightings, no Loch Ness monsters or sea serpents have ever been found. But in the 1600s, the specimen of a curious long-necked creature emerged that could explain where such aquatic tall tales may have originated - if only it hadn't been mislaid.
In the late 17th century, the botanist Nehemiah Grew published a catalogue of oddities held by the Royal Society in London. The book, called Musaeum regalis societatis, contains the first scientific description of a skin belonging to an unusual seal. He writes: "Wherein he principally differs, is the length of his neck; for, from his nose-end to his fore-feet, and from thence to his tail, are the same measure." By contrast, most seal necks are only about a half the length of their lower body. In 1751, Grew's description was cited by James Parsons in the Royal Society journal Philosophical Transactions (vol 47, p 109). Parsons included it in his list of known species.
Nobody has seen the skin since, and no further specimens have emerged. Could long-necked seals really exist? The idea persists but is now relegated to cryptozoology, the search for semi-mythical species. Cryptozoologists argue that many legendary creatures have actually existed and point to the colossal squid or king cheetah as examples."
The problem is the infrequency of appearances for a mammal which should surface often to take on air. There is also the small matter of the seal's penchant for coming ashore to do some basking. Now I don't doubt there are ways around this. For example, evolutionary theory dictates that scarce resources employed in wasteful ways are candidates to be naturally selected away. Thus, when an animal approaches the surface, it is only needful to employ energy to expose the required air intake orifices. In other words, only the nostrils need be exposed and not the entire neck.
It is also entirely possible this is a form of pinniped that does not take to shore much. But for me the behaviour of Nessie points away from a primary air breather (though since it has been seen on land, some form of air respiration is employable).