Saints nearly gifted the visitors a goal in the opening moments when keeper Daniel Petetz passed the ball straight to Watford’s Edo Kayembe on the edge of the box but the shot that followed was a tame effort.
The hosts had their best chance just before 20 minutes, Flynn Downes crossed a ball into the area from the right which found its way to Ross Stewart in a central position but his shot was saved by Watford keeper Egil Selvik.
There was a pause in play after half an hour as Flynn Downes and James Abankwah came together in a nasty-looking collision.
The Southampton midfielder came off worse out of the two players and then spent a lengthy period of time on the ground before receiving treatment. He tried to continue but was brought off before half-time despite his best efforts.
Waford fans wanted a red card for Taylor Harwood-Bellis in the second half as he brought down Mamadou Doumbia who looked ready to burst in behind, but referee Elliot Bell produced just a yellow for the defender.
Saints finally broke the deadlock when James Bree swung a corner into the box from the left and his delivery found the head of Larin who powered the ball home just moments after being substituted on.
The home support were then signing his name in the moments after the goal, delighted at his contribution.
Southampton were denied a penalty in added time as Sam Edozie looked to be wrestled to the floor in the box but referee Bell waved away the appeals.