During my "research" this weekend, I came across a very interesting post by Dr. Marcos Asensio in the blog Animal de Ruta . In this post he proposes that Nahuelito is the result of natural gas and oil seeping out of faults that lie below Lake Nahuel Huapi's bed. It is an excellent theory and it adds yet another natural cause to all the other ones that can explain the Nahuelito myth in a rational manner.
Oil and Gas
By a creek now known as Las Minas which was known as Pichi Ñireco, there is a natural seepage of oil. It is located about 25 km ( 15.5 mi.) south east of the city of San Carlos de Bariloche, within a private property (La Lucha) close to the Nahuel Huapi National Park (see red circle in map above).
This natural well was discovered ca. 1890-1900 and it was explored between 1916 and 1922 by the Compañia Minera Chileno-Argentina who drilled four wells here. Later in the 1930s, YPF sank two more wells. It was followed by La Celina (later named Isaura) in the 1940s and, between 1982 and 1984 it was surveyed by Pluspetrol and Exxon but they did not attempt to exploit the oil because it is not "mature" for use as fuel.
Recently, in 1997, Emerald Energy showed some interest in the Ñirihuau basin but did not do anything in the field.
In 2008 a permit was granted to the joint venture by YPF-Pluspetrol to explore transport and sell oil from the Ñirihuau basin. But oil still seeps out and natural gas bubbles out of it. The oil is bituminous, a heavy crude with low density that looks like tar insoluble in water. 
The provincial Wate Department did some primary containment works with poor results during 2008 trying to reduce the pollution.
Lake Nahuel Huapi occupies a depression formed by several tectonic faults that was later deepened by the glaciers during the Ice Ages. We will point out the two main faults: Otto Fault, which separates the Ventana and Ñirihuau formations, which are both part of the Nahuel Huapi Group. These are Tertiary rock formations. The Otto Fault crosses Challhuaco stream, Mount Otto, Campanario Fjord, San Pedro Peninsula (in blue in the map above) and along the Lake's western side to its northern tip.
Catedral Fault is similar in extent and trajectory going from the Pichileufu River by Mount Millaqueo, Rincon Fjord along the western shore of the lake. Shown in red in the map. It is not as straight as the Otto Fault.
There are other lesser faults marked in black in the map.
Faults, oil and Nahuelito
Asensio  suggests that most of the shipwrecks on the lake have happened in the area between Victoria Island ("4" on the map) and Peninsula San Pedro (“5”), this area is marked "3" on the map. Asensio mentions some ships that have sunk here such as the “Wapi” in 1989, the “Entre Ríos” in 1993 and the “Therapia” in 2006  and says that they may have been caused by gass that vents through the fault.
As gas is constantly escaping from the seepage by Minas Creek, this shwos that the oil field has plenty of gas. The fault under the lake is very deformed and this traps and retains the gas that is released from the oil increasing its pressure till it reaches a point where it can break free and escape by degassing.
This means that it has to overcome the pressure of several hundred feet of water column and the lake bed sediments.
The released gas will appear as bubbles on the surface of the lake and will also have some hydrocarbons. This could cause ships to sink if they happened to sail across a surging plume of bubbles.
The reason for this is that water would become less dense due to the bubbles and the ship would ride deeper and deeper in the water to keep afloat in a medium with lower density (Archiemedes Principle), sinking.
There have been dozens of shipwrecks on the lake and most are due to the terrible storms that rage with great violence. For instance the "Helvecia” in 1907  , a steel ship 17 m long (55 ft.) sank during a storm after "a gigantic mass of water sank it ". Personally I have sailed on the lake with my father while fishing by trolling, and the crossing from Victoria Island to San Pedro as well as the northern sheer rocky face of the Peninsula are exposed to the waves pushed by the strong western winds. The area where the Blest and Tristeza fjords meet the lake's main body is known as the "Cockatil Shaker" due to the strong waves that clash there.
Nevertheless, it is quite possible that a sudden mass of surging gas could sink a ship.
Is Monstrous Nahuelito only gas?
More interesting than capsizing ships is the fact that a sudden irruption of bubbles can make the lake's surface move in an unexpected manner and the casual onlooker would believe that it was caused by some strange animal. Furthermore the "oil slick" with the oil that surged with the gas would alter the lake's color and induce people to believe that they were seeing the dark back of an aquatic being.
Due to the length of these faults they could easily explain sightings that occurred at Campanario Fjord ("2" on the map) or in front of Bariloche city where two faults meet. Also at Huemul Fjord ("1" on the map).
Sightings such as the Strange blotch or the ones mentioned in my post on Nahuelito (including the video at Campanario), can be explained by bubbles causing froth in the lake.
Part 2 of Nahuelito Bubbles and Gas
 Marcos Asensio, (2010). La leyenda del Nahuelito. 14.08.2010. Ñirihuau hoy: contención a medias de manaderos de petróleo. 19.10.2009.
 Bernardo, L. M., Zamora, G., Folguera, A. y Zapata, T., (2009). Nuevo Modelo Exploratorio y Definición de los Sistemas Petroleros de la Cuenca del Ñirihuau, Cordillera Patagónica, Argentina
 Quiroga J., Rolando Granada J., Formulación del Informe Ambiental del Centro Atómico Bariloche
 Clarín. 27.10.2006.
 Histarmar. La primera tragedia naval del Nahuel Huapi
Monstruos de la Patagonia - Criptozoologia, Mitos & leyendas de la Patagonia 2010 Año Internacional de la Diversidad Biológica Copyright 2009-2010 by Austin Whittall ©