This is not technically a Patagonian being. It was not sighted within Patagonia, it was seen a few hundred miles north of Patagonia, on the southern sea shore of Buenos Aires Province in the South Atlantic Ocean’s Argentine Sea (see the map above).
However, since I have mentioned a Sea Monster in the Strait of Magellan and another in the River Plate, maybe "Joselito" can throw some light on these sightings.
It was reported at Necochea in 1993 and 1994. I will summarize below what has been written about it. The main online source is an Investigative Report by Guillermo Daniel Giménez.
There was a sighting of a “sea monster” on the 18-19th of March, 1994, about 6 miles (11 km) from the port of Necochea, Quequén by the “Arenas Verdes” area. It was seen by some fishermen on board the “Paco Ventura” owned by Mr. Carlos Miño.
The creature was of a dark brown - black color “with an enromous head, very big eyes and its neck and back with a serrated edge”
The creature had been seen about six months earlier in the same place according to Fernando Bertoia, who in turn reported that the creature was 10 to 12 meters long (32 – 39 ft.).
This, first sighting involved the crews of the fishing ships “Macho” and “Deleite C”.
Luis Ramón Menna, of the “Paco Ventura” said that in his 25 years as a fisherman, he had the chance to see whales, killer whales but never something like the creature that he had seen: “I did not see it all, it had like ‘saws’ on its back, it was dark”. He could not define its size, but believed that it was big due to the waves it made.
Menna also recalled that in 1993, a person who had been camping on the shore close to “Medano Blanco” had seen, with his binoculars, two “gigantic marine creatures that seemed to be playing banging their long necks together”
Local fisherman, Salvador Lucero Melluzo drew a creature that he had seen about two months earlier, and it appeared to be similar to what the crew of the “Paco Ventura” had seen. He said that “it was dark, very big… its legs were like very big paddles, like elephant ears”
During this period, fish were scarce and though some blamed this to the jelly-fish, others blamed the sea monster.
It was named Joselito because it was officially sighted on March 19th which is the day of “Saint Joseph” (in Spanish: San José) hence the diminutive: “Joselito”.
According to Carlos and Fabián Miño, owners of the boat, the creature was not agressive and “ignored” them. There “were sea gulls around it... a while later it submerged… and then it stayed there floating.
It had “fins … [like] the dinosaurs that we see in books or magazines… its head was white, rounded, like a duck’s head”. Its neck was “long… like that of a swan, but much bigger”.
Fabián Miño added that its "back had the shape of a crocodile and was about 4 meters long and over 2 wide (13 ft and 6.5 ft. respectively).
On May 3rd (year not stated, 1993? 1994?) María Susana Monterosa and Osvaldo Gutiérrez who look after several homes at Bahía de los Vientos, saw a strange sea creature close to the shore. It had: “A big head and a dark back, like a saw, it had big eyes… it was 3 or 4 meters long (9 to 12 ft.)”.
Up to here what Giménez has written about this creature.
There is a photograph of it on the web, that actually looks like a reef:
1. A Sea turtle.
The creature may be a Leather Back sea turtle. The local museum (Museo De ciencias Naturales Dr. José Escuadrone) in its website, says one was caught by some fishermen in 1995 . It has a photograph, which we show below:
This is a very rare animal, and quite uncommon in the South Atlantic. Furthermore, they are quite scarce in Argentina's sea coast, the water is temperate to cold, not quite their habitat.
However, there have been sightings along the Buenos Aires sea cost, including Necochea., one was washed up on the shore at Miramar close to Necochea in 2005 and another in 2006. 
New. December 10th, 2009: I found some fresh information on turtles, and posted it Here.
2. A giant squid.
As the fishermen who saw "Joselito" said they had seen turtles, killer whales, whales and other fish, the creature they saw must be something extremely rare, perhaps a giant squid.
These mysterious beings of the deep have been caught in the Argentine sea before: but not close to Necochea (one was caught by Isla de los Estados near Tierra del Fuego Island), it was 3 meters (9 ft.) long and weighed 150 kg (over 300 lbs) this happened in 1999, and there are only two previous records of this kind of catch in these waters, in 1998 and 1995 
A Giant squid also appeared on the Southern Patagonian sea coast in 2008 there is a photograph of it lying on the beach, which I reproduce below :
So, there are several plausible and rational explanations for "Joselito". Personally I don't think that it is a sea serpent.
Another alternative is that Joselito may be an Oarfish a "sea serpent" (actually a fish that looks like one).
 Giménez, G., D. Extrañas criaturas marinas en Necochea y Quequen. Online Here.
 Ecos Diarios. 09.Aug.2007. Online Here.
 Museo de Ciencias Naturales Dr. José Escuadrone. Online Here.
 Same Source, Here.
 Martínez, D. E.,(1984).The Loggerhead Turtle in Bahia Blanca, Argentina, Chelonidae. Historia Natural. Vol 4. No. 21. Online Here.
 Online Here.
 La Nación, Buenos Aires (23.03.1999). Capturaron un calamar gigante.
 Estancias de Santa Cruz. Online Here. (Image is Here.)
 Instituto Geográfico Nacional de la República Argentina. Mapas Escolares. Online Here.
Copyright 2009 by Austin Whittall ©