There are many stories of drowning humans being rescued by Dolphins. Are these just tales or does this actually occur?
I found this in the International Journal of Aquatic Research and Education
The use of dolphins in rescuing humans has been instinctive, rather than the result of formal training. Dolphins have rescued drowning victims or abandoned survivors by allowing them to ride on their back (Betancourt, 1998; Catton, 1990) or by psychologically encouraging them with their presence to continue their efforts to reach the shore (Waga News, 2001).
The paper cites many evidence of dolphins helping drowning human (which can be found in the references a the end of the paper). However, the study also claims that animals can either do it by training or by instinct (depending on the breeds).
Therefore it seems that animals can rescue humans either by instinct or by being trained for rescue.
Other papers can be found supporting the evidence of dolphins having helped drowning humans through history:
An important side note should be expressed. Evidence of dolphins helping drowning humans does not mean that dolphins would save a human in case of drowning. There are also some evidences of them attacking human or letting them drown.
Here is a link to a book about Dolphins The dolphin in history papers delivered by Ashley Montagu and John C. Lilly at a symposium at the Clark Library, 13 October 1962. Published 1963 by William Andrews Clark Memorial Library, University of California in Los Angeles .
A portion of the text read,
Is there any evidence that dolphins save drowning swimmers ? There is.
In 1945 the wife of a well-known trial attorney residing in Florida was saved from drowning by a dolphin.' This woman had stepped into a sea with a strong undertow and was immedi- ately dragged under.
The same book is available here as well.
Hope this helps.