The 29-year-old, who carded a round of 59 in January, began the final round with a four-stroke lead.
But he put his tee shot into the water at the 16th, made a double bogey and was tied for the lead at 14 under.
However, Hadwin parred in for a level-par 71 and Patrick Cantlay bogeyed the last to finish second.
Open champion Henrik Stenson was the top-placed European, finishing six shots back after a 70.
Northern Ireland's Graeme McDowell had five birdies in a 67 and was nine shots adrift, while 41-year-old Ian Poulter, playing on a medical extension having missed four months of last season due to a foot injury, finished level for the tournament.
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Canadian Adam Hadwin survived a late scare to claim his maiden PGA title with a one-shot win at the Valspar Championship in Innisbrook, Florida.