Australia had arrived in the United Kingdom in May. A group-stage exit in the ICC Champions Trophy was followed by a 3-0 loss in the Ashes, but Watson, who was involved throughout the tour across all three formats, helped his side end on a good note.
Watson’s innings took Australia to 298 all out, and together with Michael Clarke (75), he shared a fourth-wicket partnership of 163, on a ground where they had both once played for Hampshire. That partnership resurrected the innings after from a precarious 48 for 3 in the tenth over.
Ben Stokes, 22, the Durham allrounder, helped England pull things back after the Watson-Clarke stand, ending with 5 for 61 – his first five-wicket haul in ODIs.
Chris Jordan, who made his debut in the match after Steve Finn was ruled out with a side strain, was England’s second-most successful bowler, and took 3 for 51.
Boyd Rankin kept things very tight, but the other bowlers weren’t as penetrative, and eventually Australia ended up with a total that was good enough, even though it had looked good for a 300-plus score when Watson and Clarke were batting.
Australia will face England in the return Ashes from November. Clarke, the Man of the Series, described the win as an important one, but said it was separate from the return Ashes.
"I'm extremely happy to win the series," said Clarke. "To a lot of people this ODI series does not mean much, but to the Australia players it means a lot. I don't know whether it will have much impact on the Ashes back home, but we get the flight back home tomorrow a lot happier as a one-day side certainly.
"The Ashes is still a way away but everyone is excited about that and the players are thinking about it already, but our focus has to be on the seven ODIs in India first."
Watson, the Man of the Match, said, "Things have changed for the better in a big way. We are playing the brand of cricket we enjoy and everyone is very excited about what lies ahead."
England was led by Eoin Morgan in the series, with several senior players including Alastair Cook, the regular skipper, given a break. Despite the series loss, Morgan said the young England side had done well. "We prioritise Test cricket and with back-to-back Ashes series we have to be picky and choosy. But the next best guys here have done really well," said Morgan.
England’s chase had lost steam early when Kevin Pietersen was run out without having scored, after being slow to answer Michael Carberry's call.
Carberry, on his home ground, made 30 before he was lbw to James Faulkner, who eventually ended with 3 for 38.
With the openers gone, England was 50 for 2, but Joe Root and Luke Wright fell quickly as the score slipped to 68 for 4.
Morgan fell for 30 when stumped by Matthew Wade off Adam Voges, the left-arm spinner.
England's hopes rested with Ravi Bopara and Jos Buttler, whose 65 not out had helped level the series with a three-wicket win in Cardiff on Saturday.
Buttler was going well once again, but Faulkner bowled him for 42, ending a 92-run stand with Bopara. Faulkner’s change of angle, coming round the wicket, outfoxed Buttler.
Bopara made 66 before falling, well caught at extra cover by Voges off Mitchell Johnson, as England looked down and out at 197 for 7. Bopara was Johnson’s second wicket of the match, and his 200th in ODIs in his 129th match. Johnson completed a good series with 2 for 21 in ten overs.
Rankin was the last wicket to fall, bowled by Fawad Ahmed, as England were all out for 249 in 48 overs.
Earlier, Australia had gone from 48 for 1 to 48 for 3 when Stokes took two wickets in two balls after a 30-minute rain break.
Stokes dismissed Finch, caught by Morgan at point, and Wade, caught behind by Buttler, off successive balls. Clarke then survived the hat-trick ball, and settled in quickly.
Clarke had been a doubtful starter on the eve of the match, with a recurrence of his long-standing back injury, but he was passed fit for this match.He followed his hundred in Australia's 88-run victory in the second ODI in Manchester with another fine innings before slicing a pull off Jordan to Pietersen at mid-off to end a 76-ball knock including ten fours and a six.
Watson's dominant 107-ball innings, featuring 12 fours and six sixes, finished when he was caught behind off Stokes.