The Hornets had a great chance to take an early lead when Odion Ighalo's ball found Troy Deeney, but the striker could only see his goal-bound shot blocked on the line.
However, 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.
The Hornets were looking for a penalty shortly after their second but the official waved away the appeals after Ighalo had gone down under a challenge from Tal Ben Haim.
Ighalo then gave Watford a 3-0 lead at the end of the half when he tapped home after a clever cut-back.