I know there's some truth to the negative ion theory... it's used as therapy for some types of mood disorders. But I highly doubt that the natural concentration is high enough at the beach to create those same effects. I know that in one test, low concentrations of negative ions yielded NO results... low being about 10,000 ions per cubic meter of air. Only very high concentrations (<2.5 million ions per cubic meter of air) in controlled environments prove effective.
Still... it can't be bad for you, that's for sure!
endorphins released from simply exercising are nowhere near the endorphins released when surfing. plus, there's a sustained elevated mood that occurs (with me, at least) for the entire day into the next day after a really good session or from bigger waves. i can't compare it to exercising. there's more to it than that. that's why a word was created for it called "stoke". imagine what those dudes feel like who conquer Maverick's or Chopes for a day feel like? that's why they are junkies.