There is a fossil seal, the Acrophoca longirostris (the second part of its name means 'long face' in Latin), which lived in the Pacific Ocean by the coast of Peru and Chile during the Miocene and Pliocene periods (23.3 to 2.5 Million years ago). It has been described as a “swan-necked” seal.
As most 'lake monsters' are described as having 'swan necks', this seal if still alive, would be the ideal candidate to fill in the lake monster's shoes. Lets get the facts:
First the bad news: according to paleozoologist Darren Naish, they were not so long-necked; thought they would “have looked longer in the neck than any extant seal”. These were not mammalian "plesiosaurs".
However Acrophoca had longer cervical vertebrae and a cervical column (neck) than modern extant seals.
Their neck measured 32.9 cm (or approx. 1 ft 1 in), while regular monachine seals’ necks are about 21.8 – 24.9 cm (8.6 – 9.8 in.). To express this in another way, the neck of these Acrophoca represented 21% of the length of their vertebral column while in other terrestrial carnivores and seals it is about 17-19%. Not much of a difference.
Based on this, Naish concludes that “sadly, ‘swan-necked seal’ really is a bit of an exaggeration”.
Regarding its placement within the “seal” family it is a hot topic among seal experts and some have suggested that it is a lobodontin and as such belongs to the group that includes the leopard seal (Hydrurga), the crabeater seal (Lobodon) and Ross’ seal (Ommatophoca).
If this was the case, we should look at these living lobodontins to get an idea of their behavior and appearance. Leopard seals are big carnivores (males can measure up to 3.3 m long and weigh close to 450 kg -11 ft and 1,000 lbs.) It is the top predator in its environment with formidable jaws and canine teeth 2.5 cm (1 in.) long. It feeds on penguins, squid and seals of other species.
Naish and Stig Walsh described some remains of Acrophoca discovered in Chile in 2002, which had longer skulls than the A. longirostris. Perhaps more fossils may indeed turn up and give us a clearer picture of this group of seals and its evolution.
From a cryptozoological point of view, this finding (which came from the same site as those of the "walrus", sea cows Odobenocetops, and the giant sea sloths, Thalassocnus) is very interesting, as a long-necked seal could readily explain many "lake monster" sightings in Patagonia.
Long-necked seals. Some facts
Besides the 'real' A. longirostris, there is another "long-necked" seal, one which despite being described by scientists in the seventeenth and eighteenth century, has not been seen again, and therefore remains a mystery. This creature as depicted in the images (above and below) had a very long neck unlike anything seen in other seals.
The image below (adapted from ) shows two seals, one, above is the enigmatic “Long neck’d Seal, or Sea Calf”, and the one below is “The common seal”. I have included a seal skeleton for comparison purposes . But first, lets go to the story.
James Parson wrote a paper in 1751 in which he described five “species” of Phoca, among them he mentioned a Dr. Grew’s “long neck’d seal” from an unknown locality. This peculiar seal was actually part of the Royal Society’s Museum, and as such it was included in a Catalog published in 1681, where it was described as follows:
From his nose end to his fore-feet, and from thence to his tail, are of the same measure 
Also, “instead of his fore-feet, he hath rather fins; not having any claws thereon, as have the other kinds.” 
Parsons described a ‘young´specimen which measured 7.5 ft. (2,28 m) long. We ignore the size of an adult. However, as can be seen above, the specimen was 41% longer than the 'common' seal and its neck and head represented 43% of the total body length (which coincides with Grew's remarks of 50/50 relationship of nose to fore feet and tail to forefeet). In the common seal, the head plus neck is only 26% of the total body length. Indeed it is a very big "long-necked" seal.
What kind of seal was it? It is generally considered as either a mythical or an indeterminable species. its scientific name, as given by Dr. Shaw in his Zoology(1800) is Phoca longicollis or long-necked seal. 
Allen  contends that the shape of its front feet and its longer neck, make it an "Eared Seal", or an "Otaridae". However Fischer (1827) in his Synopsis) places it with the lobodontins: the Sea-Leopard of Weddell which is an Earless Seal or a "Phocidae".
Allen also asserts that its habitat though unknown must have been either the Cape of Good Hope or Southern South America because no seals from Australia or the North Pacific reached England before 1686. And states that it may have been a Sea-Lion (Otaria leonina). These are "Otaridae" and have visible ears.
By the way,in 1670 Sir John Narborough explored Patagonia from Puerto Deseado on the Atlantic to Valdivia, in Chile, went through the Strait of Magellan twice and spent part of a winter at San Julián. He could have brought the seal with him. Before his voyage we can only mention Cavendish's and Drake's expeditions, but they were more interested in plundering Spanish riches than describing the native fauna.
The Patagonian link
So here we have an earless long-necked seal which may have lived in the Southern reaches of the South Atlantic Ocean, similar to the "sea lion" which can be found on the coasts of Patagonia.
We also have fossil evidence of a "swan necked" seal in Peru and northern Chile, which despite being far from Patagonia, places these creatures in the same part of the globe.
The "long necked" seal is described in Gronovius (1760) Bibliotheca as having a "capite lutrae", that is "with an otter head" (Bold, mine). This detail combined with the possible geographic location, its long neck, lack of ears and size make it a likely candidate to embody our mysterious Patagonian iemisch or water tiger, or perhaps our Strait of Magellan Sea Monster.
Now the bad news: we should bear in mind however that iemisch had clawed paws and a long tail. the long-necked seal lacks both ("not having any claws"). So, perhaps some other species within the group could account for our iemisch.
Piling speculation on speculation we can also imagine that it may even have adapted to freshwater (like the landlocked Lake Baikal seals Pusa sibirica or those at lakes Ladoga adn Saimaa have done in Russia) and running upstream from the Atlantic or the Pacific Oceans, made its home and lived in the Patagonian lakes until quite recently. All this of course is wild guessing.
In my book, based on etymology and some comparisons of the words used by the Mapuches and different Tehuelche Groups to name seals and other aquatic animals, I came upon the possibility of a freshwater seal in Patagonia. Below is an excerpt from my book on this intriguing subject:
Note: Above, the sea wolf (South American Sea Lion) is also known as Lobo Marino de dos pelos) Otaria flavescens. The "sea lion" mentioned by Allen (Lobo Marino de un pelo) or Otaria leonina is also known as Otaria Byronia.
 Darren Naish, (2006). http://darrennaish.blogspot.com/2006/02/swan-necked-seals.html> target="_blank" title="External link">Swan-necked seals. Tetrapod Zoology. 04.02.06
 Darren, Naish, (2008).
The Long-necked seal, described 1751. Tetrapod Zoology. 25.09.2008. The image’s original source is Tab VI in: Parsons, J., (1751). A dissertation upon the Class of the Phocae Marinae. Philosophical Transactions 47, 109-122.
 Gaston Bonnier Squelettes de phoque et de dauphin
 Nehemiah Grew, (1681). Museum regalis societatis or a Catalogue et description of the natural and artificial rarities. W. Rawlins, Ed. pp. 95.
 Michael A. Woodley, Darren Naish and Hugh P. Shanahan, (2009). How many extant pinniped species remain to be described? Historical Biology: An International Journal of Paleobiology. 20(4):225-235.
 Joel A. Allen, (1974). History of North American pinnipeds. Ayer. pp 214.
 Gronovius Laurentius, (1760). Bibliotheca regni animalis atque lapidei, seu, Recensio auctorum et librorum : qui de regno animali & lapideo methodice, physice, medice, chymice, philologice, vel theologice tractant, in usum naturalis historiae studiosorum pp. 203
Muizon, C. de, (1981). Les vertébrés fossiles de la formation Pisco (Pérou). Première partie: deux nouveaux Monachinae (Phocidae, Mammalia) du Pliocene de Sud-Sacaco. Travaux de l’Insitut Français d’Études Andines 22, 1-161.
Walsh, S. A. and Naish, D. (2002). Fossil seals from late Neogene deposits in South America: a new pinniped (Carnivora, Mammalia) assemblage from Chile. Palaeontology 45, 821-842.
Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia2010 International Year of Biodiversity Copyright 2009-2010 by Austin Whittall ©