Watford were again denied by clearances off the line with Gabriele Angella and Gianni Munari thwarted in quick succession.
© Getty Images
However, Craig Cathcart finally gave them the lead on 15 minutes when he volleyed an effort into the roof of the net.
Deeney, who was making his 200th appearance for Watford, notched his 70th goal for the club when he poked home from close range after 25 minutes.
Ighalo then made it 3-0 at the end of the first half before claiming his second of the game with a header in the second period after some slack marking at a corner.
Deeney thought that he had his second of the afternoon on 73 minutes, but Neil Etheridge made a great save.
Daniel Tozser did eventually make it 5-0 in stoppage time to condemn rudderless Charlton to a hammering.