Watney, who finished second at Doral two years ago, birdied the 18th to make sure of victory with a five-under round of 67, securing his third PGA Tour win. Johnson had eased the pressure on Watney with a bogey on the 16th but the winner showed his steel with an aggressive approach to the final hole where he found water on Saturday.
Dane Anders Hansen and Italian Francesco Molinari were tied for third place while Britain's Luke Donald, who began the day two shots off the lead, had to settle for a share of sixth place with Australian Adam Scott.

Watney's putting was particularly impressive with him saving par with long efforts on the par three 13th and 15th holes.

World number one Martin Kaymer had a disappointing day, fading out of contention with three bogeys before slumping to a tie for 24th after finding the water on the 18th and making a double bogey.

Tiger Woods equalled the best round of the day with a six-under 66, earning him his first top-10 finish in a PGA Tour event since the US Open in June. Woods's performance will give him a real boost ahead of his final pre-Masters tournament at Bay Hill later this month.