Bournemouth made a good start and raced into a three-goal lead through Sophie Quirk, Zoe Barratt and Amber Treweek, before Madison Perry got a goal back for the Hornets to see Helen Bleazard’s side lead 3-1 at the break.
The visitors kept their hosts at bay well, but it looked set to be a nervy finish when Flo Fyfe made it 3-2 with five minutes to play.
But Bournemouth completed the victory when Quirk was fouled in the box and Kenni Thompson converted from the penalty spot to make certain of the points with the final kick of the game.
That saw the Cherries round out a successful first campaign in the National League Southern Premier Division with a third place finish on 46 points.
The Cherries were out to secure the league double over their hosts and they made the best possible start.
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Just three minutes into the game, Barratt played a perfectly weighted chipped ball over the top of the Watford defence for the onrushing Quirk, and she lifted the ball over keeper Sophie Hillyerd to give the visitors the lead.
Perry had the hosts’ first effort on goal but it was tame and comfortable for Katie Scadding in the Cherries net.
Some excellent scrambling defence from Bournemouth kept their lead intact, as Coral Haines was slid in on goal, Kelci Bowers raced back and got to her before she could shoot, she teed up Poppy Wilson but her well-hit effort was straight at Scadding.
Moments later, it was 2-0 to the Cherries.
Quirk and Barratt again connected, and this time it was the ex-Portsmouth forward who set up her teammate.
She did well to get away from her marker and to the byline, where she cut the ball back for Barratt, and she brought it under control and smashed home.
Kelci Bowers (Image: Richard Crease)
Watford sought a way back into the game but the visitors defended well, surviving a string of three corners in quick succession.
Treweek tested Hillyerd with a powerful effort but she kept it out, before the Cherries missed a golden opportunity.
Quirk crossed from the left, it was deflected and Hillyerd couldn’t claim, it came to Jenna Markham but the deflection did for her and she couldn’t keep her effort down.
Markham headed over the bar from a Kenni Thompson free kick, before the Cherries went three in front.
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Molly Barron-Clark swung in a corner to the far post, where it was met by Treweek, her header was saved on the line but it came back out to her and she powered into the back of the net.
Freya Meadows Tuson saw her header comfortably caught from another Thompson free kick.
The hosts got a goal back after 38 minutes, when the ball came to Perry at the far post, she brought it down and finished past Scadding.
Bournemouth took their two-goal advantage into the break, and they went close to extending it in the opening exchanges of the second half.
Barron-Clark slid in Thompson on the left and she powered towards goal, it was well saved but Bowers wasn’t far away from touching home at the far post.
The visitors were pressing in the opening exchanges of the second 45, and went close in a goalmouth scramble from a corner but Meadows Tuson couldn’t poke it over the line.
Quirk shot off over the bar and then wide as the forward was a real threat to the Hornets.
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Bleazard went to her bench after 65 minutes, as Katie James and Georgia Wilson replaced Barron-Clark and Elena Cole, before Phoebe Williams replaced Markham eight minutes later.
Perry fired towards goal but it was comfortable for Scadding.
Bowers and Treweek were the next to be replaced, as Charlotte Harris and Sophia Leonard were introduced with 13 minutes to play.
Thompson curled a free kick just wide of the top corner before the hosts got a goal back, when Fyfe curled into the back of the net from the angle on the left with five minutes to play.
Harris cleared off the line from a Filbey effort and it was backs to the wall defending from the visitors as they fought to combat a late surge from the hosts.
The Cherries sought to take the ball to the corner as the game entered stoppage time, but as the time ticked down, Quirk got beyond her marker and surged into the box, where she was fouled.
The referee pointed to the spot and Thompson confidently converted in the final play of the game, to see the Cherries end the season on a high with a 4-2 win over the league champions.
That concluded a successful first campaign in the third tier for Bournemouth, including silverware in the form of the National League Cup.