We coud ask Elliott, his CO, if this was ever the case or ADM Hoffmann. Kerry wasn't a free agent. He just couldn't take his boat and crew and go on a secret mission without his boss knowing. It just doesn't work that way.
There's another thread going that quotes Adm Hoffman. In it he states that he NEVER authorized any such 'secret mission' into Cambodia - and he would have had to 'sign off' on one. And as such - the Adm states as fact that Kerry was never in Cambodia and is lying.