Playing the first of their 10 matches only four days after arriving, the tourists never really got on a roll against a committed Barbarians team who gave them a reminder of the depth of quality in the New Zealand game.
"It was a tough game. They really took it to us and put us under pressure," Lions coach Warren Gatland said. "We'll get better after the hit out.
"The guys are still recovering after arriving on Wednesday, but definitely something for us to work on."
Lions flyhalf Johnny Sexton slotted a first half penalty, scrumhalf Greig Laidlaw then stepped up when he went down injured to add a second, and replacement flyhalf Owen Farrell converted Watson's try.
Captain Sam Anderson-Heather scored the only try for the Barbarians and suffered injuries to his knee, ribs and hand before he was replaced at halftime.
Flyhalf Bryn Gatland, the son of the Lions coach, converted Anderson-Heather's try and showed he would be more than capable of playing at a higher level should the Auckland Blues need him on Wednesday against the tourists.
More than 30 hours of non-stop rain had swamped the region, but the match was played in relatively dry but slippery conditions in front of a sell-out crowd of 19,951.
The Lions had been expected to withstand an early onslaught before grinding down the side composed of players from New Zealand's semi-professional provincial competition.
Despite the fact Gatland had chosen the side before they left Britain and they spent a week training together, though, the Lions lacked cohesion and timing.
Captain Sam Warburton said that would come as they built into the tour.
"Not too bad given we have only come together so quickly," he said. "But definitely plenty for us to work on.
"They tested us and it's better to have that than a 50 point game. ... Every time we play there will be things we need to try and find improvements."
The Lions had tried to bludgeon the Barbarians into submission, but once they realised they were solid defensively, they began to seek space and keep the ball alive.
The pressure began to pay dividends as the game wore on and they were held up over the line on several occasions, which gave the hots a 7-3 lead at the break.
That momentum continued in the second half as the Lions stepped up their defensive effort and kept pressure on the home side, but they were unable to score.
It was ultimately their defence that led to Watson's try after a superb driving tackle by Alun Wyn Jones set up the field position and allowed them to hold the ball for a series of phases.
Wales blindside flanker Ross Moriarty, one of the few Lions players to stand out, ran into space but was brought down just short and Farrell passed to Watson to cross in the corner.
(Reporting by Greg Stutchbury; Editing by Nick Mulvenney)
