This isn't a fond war memory gone wrong. It's a politically motivated lie that was found out.
When this story first popped up a day or two ago, I read somewhere (probably on the FreeRepublic) that several boats had inadvertently strayed into Cambodia earlier. To prevent a recurrence, several boats were posted at the border, and concrete pilings were driven into the river bed.
Therefore, Kerry could not have been in Cambodia as a result of a navigational error.
The only alternative to rescue this story is to be on a secret mission with the CIA (yeah, the CIA, that's the ticket - the yahoos will fall for that one!)