In order for this story to be true, the following must be true: 1. Dick Van Dyke, 84-years old, still surfs.
Plausible, I guess.
2. Dick Van Dyke fell asleep on his surfboard and drifted far enough into the ocean to lose sight of land.
Possible, depending upon the size of the swells and the amount of alcohol that Van Dyke had consumed just prior to surfing.
3, Dick Van Dyke was eventually awakened by a school or porpoises, which he initially mistook for sharks, and they pushed him back to land.
Dolphins and porpoises have been known to rescue drowning swimmers from time to time, so I guess this might also be possible.
The thing that makes the story unbelievable is the unlikely confluence of events:
The 84-year old star of Mary Poppins and Chitty Chitty Bang Bang manages to fall asleep on a surfboard long enough to drift offshore, only to be rescued by a school of marine mammals.
Each fact alone is hard enough to believe. Stringing them together makes the whole situation seem absurd.
Not worthy of even the most fantastic of fiction.