The visitors struck first when Mike Brown blasted his way between two defenders after four minutes. The Boroughmuir centre doubled his tally when he took a scoring pass from Tom Drennan who had profited from indecision in the home ranks to pounce on a loose ball.
Watsonians had struggled make an impression but they did eventually find a way into the game and cut the deficit when Jamie Hodgson dotted down in the corner. And by half time, the scores were level after Euan Dods barged over.
Watsonians survived a spell on the back foot before taking the lead in 51 minutes when Mark Bertram who darted over. Andrew Chalmers added the conversion.
The bonus point was secured when Rory Hutton produced a sublime solo effort that left Chalmers a simple conversion. The home full back then completed a triumphant afternoon for Watsonians when he thumped over a long range penalty in the final minute of regulation time.