Sunderland are looking increasingly doomed after Miguel Britos scored the only goal of a scrappy game to ease Watford's own relegation worries and leave David Moyes' men in real trouble.
Britos broke the deadlock in the 59th minute of a game that was low on quality, as befits two teams had mustered only three league victories between them in the previous three months.
The decisive goal was scrappy, in keeping with the game. Tom Cleverley's corner on the left was thumped on to the bar by the heads of first Stefano Okaka and then Sunderland's Fabio Borini. When the loose ball was lobbed in again from the left, Britos rose highest at the far post to head it over the line from close range.
It was harsh on Jordan Pickford, who had repelled Watford's best efforts almost singlehandedly up to that point. The England under-21 keeper made fine saves in the first half to keep out stinging shots from M'Baye Niang twice and Abdoulaye Doucore.
Pickford also saved well with his feet when Watford substitute Isaac Success was given a clear run at goal shortly after the home side had taken the lead. Watford were also wasteful, against a Sunderland side looking increasingly doomed. Okaka had a simple chance to score from Etienne Capoue's cross before half-time but directed his header over the bar from in front of goal.
Capoue, Doucoure and Jose Holebas all fired high or wide from long range, and when Success finally got a shot on target in the closing stages, Pickford was equal to it.
At the opposite end of the pitch, Defoe was feeding on scraps and barely had a chance, while Adnan Januzaj was half-hearted with the chances that came his way. He was replaced by Didier N'Dong, who inspired a late rally from Sunderland, and Heurelho Gomes was finally tested when he tipped away a fierce drive from Borini.
But it was too little, too late, and Sunderland remain rooted at the foot of table with time running out.