A military policy change in regards to Cambodia resulted in O'Neill, who took over Mr. Kerry's boat when Kerry returned to the United States, being assigned to a much closer patrol of the Cambodian border.
That's how I see it.
If O'Neill was sent over the border on secret missions, I wouldn't be surprised... He was in Vietnam when such missions would've taken place.
Kerry, on the other hand, had left by the time such policies were enacted. I bet he borrowed the story from an associate in the anti-war movement who is complicit in not contradicting Kerry by coming out against Kerry... Or who might already be dead, after which, Kerry felt confident using that story, but in his first tellings he did not adjust it to his own personal history, which tells how Nixon still figured in the story in its early tellings.
I like ONeill's version of the truth. It sounds a lot more like reality than Kerry's. Kerry's reads like a badly written Clive Cussler novel.
I do not believe Kerry ever went into Cambodia, as helicopters (H-34) were the vehicle of choice, both for insertions and extractions. However, we did routinely go into both Cambodia and Laos, both before and after Nixon's inaguration.