John-Joe O'Toole's winner after 72 minutes gave new Watford boss Brendon Rodgers a second successive victory.
In atrocious conditions the first-half was cautious, but three goals followed in five minutes after the interval.
Visiting keeper Keiren Westwood made some fine saves but was sent the wrong way from the spot by Tommy Smith after Elliott Ward felled Tamas Priskin.
Clinton Morrison diverted Jay Tabb's shot in to level but O'Toole nipped in after uncertainty from the defenders.
Watford manager Brendan Rodgers:
"I thought John-Joe O'Toole did exceptionally well. His goal was terrific.
"John-Joe and I have had this sort of rapport from early on and he's a good guy. I was just waiting for the right moment to put John-Joe in.
"I think you've seen today that with conditions like that you need drive, but I am also trying to get him to think about his game."
Coventry boss Chris Coleman:
"Anything can happen in conditions like that. I think that in the end it was a little bit dangerous.
"There was not really any football being played by both teams because really it was impossible.
"We said at half-time the team that makes the least mistakes in their own penalty area is going to come out on top, and we ended up giving away a penalty."
Watford: Loach, Sadler (Bridcutt 81), Bromby, Mariappa, Doyley, Williamson, Harley (Robinson 59), O'Toole, Jenkins (McAnuff 87), Priskin, Smith.
Subs Not Used: Lee, Hoskins.
Booked: Priskin.
Goals: Smith 62 pen, O'Toole 72.
Coventry: Westwood, Hall, Ward, Turner, Fox, Doyle (Best 75), Gunnarsson, Tabb, Simpson, Eastwood, Morrison.
Subs Not Used: Marshall, Walker, Clarke, Wynter.
Goal: Morrison 67.
Att: 14,075
Ref: Jon Moss (W Yorkshire).
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