There is public documentation of other secret missions to Cambodia in 1968-69. In most, if not all, cases it seems that agents and special ops were flown into Cambodia by helicopter. If boats were used, the Navy had available smaller, quieter craft than swift boats. That makes Kerry's story seem implausible, but it could still be true. If he made public the journals he provided to Brinkley, there might be more evidence that could be checked.
On the Christmas story (which even the pro-Kerry New York Times admits Kerry has not "put to rest"), perhaps Kerry was just confused about dates, or perhaps he convinced himself that an untrue story was true, as people sometimes do, and had no intent to mislead. A more unsettling possibility is that he consciously leapt the bounds of truth to make his experience seem more spectacular or to score political points. Those are not the sort of things most people want in a president.
Will Kerry's evidently untrue statements about Christmas in Cambodia raise doubts about his as-yet-uncorroborated stories about later Cambodian missions? Will they undermine his credibility and bolster the charges of his swift boat critics? Not clear.
Most of Kerry's boat mates testify to his heroism; most of those serving on other swift boats in the unit take a different view. So far as I know, all served honorably and are entitled to respectful attention -- some may have political motives, in both directions. Battlefield memories inevitably and understandably differ. But character counts in presidents, and some of Kerry's statements over the years -- not all, but some -- count against his character.