In Douglas Adams' acclaimed science fiction novel, So Long, and Thanks for All the Fish, dolphins (yes, dolphins) do mankind a great favor when, just before the Earth is demolished to make way for a hyperspace bypass, they create a replica of the Earth and transport everything to it
as a way of saving the human race. The super-intelligent dolphins, however, did not inhabit the new Earth because they had important business to tend to in an alternate dimension.
Now, this whole situation is a tad ridiculous, but it isn't entirely fictitious. In reality, dolphins have saved humans on many occasions.
In two (sort of) similar incidents, one in 2004 and one in 2007, pods of dolphins circled imperiled surfers for over thirty minutes in order to ward off aggressive great white sharks. And in 2000, a fourteen year old boy fell off a boat in the Adriatic Sea and nearly drowned before being rescued by a friendly dolphin. The marine mammal swam up alongside the boy and pushed him back to the boat from which he had fallen, where the boy's father promptly scooped him up.
Far from being merely a modern phenomenon, historical accounts show that dolphins have been saving humans for centuries. In the 1700s, a pod of
dolphins helped rescue Vietnamese sailors when their boat was sunk by Chinese
invaders. According to the Whale and Dolphin Conservation Society, recorded stories of dolphins protecting humans date back to ancient Greece.
We know that dolphins have lent humans a helping flipper on countless occasions, what we don't know is precisely why. Scientists, however, do know that dolphins are incredibly intelligent, large-brained, and highly social mammals -- like us in these respects. Scientists have also found that dolphins are capable of mirror self-recognition, a primary indicator of self-awareness. And, according to researchers at the National Aquarium in Baltimore, this capability "is thought to correlate with higher forms of empathy and altruistic
The altruism answer is certainly possible, but other theories abound. Can the dolphins' behavior be attributed to a biologically programmed response? Were the dolphins merely attempting to play with humans and saved them inadvertently?
Right now, your hypothesis is as good as any. The only way we may obtain a definitive answer is by asking dolphins, themselves.