Fortress Fullarton survived, courtesy of a brave Marr fightback in the second half of a pulsating game.
Into the wind in the first half, Watsonians made nearly all the running and Rory Hutton secured the breakthrough with a terrific solo try after 18 minutes.
The visitors maintained their pressure and, with Marr lock Ewan Bulger sin-binned, they doubled their lead in 25 minutes when, off an advancing scrum, a popped pass saw Jamie Farndale sprint down the narrow side for another unconverted try.
Marr cut the deficit when Colin Sturgeon celebrated his 100th game in a row with a penalty goal to complete the first-half scoring.
Andrew Chalmers extended the Watsonians’ lead with a 43rd minute penalty goal, but, that merely got Marr going as they took control.
Sturgeon goaled a second penalty two minutes later, before a superb drive by the Marr pack was finished off by Glasgow Warrior James Malcolm while Watsonians’ Kyle Whyte was in the bin. Sturgeon brilliantly converted from the touchline.