Swansea manager Roberto Martinez described it as "a very, very poor match" but that didn't bother Mackay.
"We showed energy, application and enthusiasm," he said. "Exactly the same as on Sunday, only we didn't give away sloppy goals."
Victory came after Watford had survived an 88th minute free-kick from Swansea substitute Jordi Gomez that rebounded from an upright.
Managers in waiting need that kind of luck, but it will take more to get the job full-time. "There are a lot of good people who also want the job," admitted Mackay.