The Black Cats' attacker produced a routine finish as the visitors swiftly worked the ball from one end of the pitch to the other three minutes after the interval.
We carved out a few openings after that, all three substitutes making an impact, but ultimately it was back-to-back defeats for us in the Barclays Under-21 Premier League following our 3-1 reverse against Liverpool on April 17.
Team news saw Milos Veljkovic start and captain the side, the central defender marking his return from an injury sustained on loan at Charlton back in February, while Nathan Oduwa - currently on loan at Luton Town - was afforded some game time out wide.
The lads struggled to find an early rhythm and Luke McGee was clearly the busier of the two keepers in the first period.
Christian Maghoma was on hand to thwart George Honeyman as he tried to break forward after letting a long kick from visiting stopper Jordan Pickford bounce into his path, while Kyle Walker-Peters had to slide in to prevent Watmore from racing clean through after our quickly-taken free-kick was intercepted and Mikael Mandron looked to start the breakaway.
We finally started to make inroads at the other end on 17 minutes when Walker-Peters headed McGee's long ball upfield into the path of Will Miller. He galloped into space centrally but held onto possession for too long and ran into Andrew Cartwright at the edge of Sunderland's area.
Miller's free-kick was steered over the bar by Maghoma as we started to take control of the possession without seriously testing Pickford - although the Black Cats nearly came unstuck mid-way through the half when Harry Winks surged forward and played Oduwa in down the left channel. His low cross-shot was too far in front of Shayon Harrison and was shepherded out by Cartwright at the back post.
Sunderland went straight back on the offensive with Walker-Peters persistently undoing the knots Watmore was trying to tie him in before coolly bringing the ball clear.
Josh Onomah was next to go close, Joshua Robson coming across to block after Harrison intelligently played him in on the right side of the box, before the visitors went closer still. A corner kick was cleared as far as Cartwright, whose rising shot from 22 yards out forced McGee into a diving save to palm the ball away.
Both sides then traded set piece opportunities, Martin Smith's curler from the left-hand edge of the box dipping onto the roof of McGee's net and Miller's right-footer from 20 yards sailing wastefully off target, before Mandron fired straight at McGee after an exchange with Watmore in the last action of the first period.
If Sunderland couldn't make the most of the opportunities they had before the break, they certainly did at the start of the second half - taking a 48th-minute lead with a simple move that started in their own box.
Pickford's clearance found Honeyman in space on the right flank and he simply lofted the ball into the path of Watmore - surging through centrally - with the Sunderland man finding the bottom corner of the net from the right-hand edge of the area.
Despite the setback we quickly regained our composure and were unlucky when Oduwa's hooked cross from the left was knocked in the direction of Winks, who volleyed the falling ball straight at the goalkeeper from the edge of the box.
Onomah was next to test Pickford, his sweeping left-footed volley forcing the Sunderland stopper to tip the ball over the bar, while the lively Kenny McEvoy - on as a substitute for Miller - skipped onto Winks' pass and found an opening wide on the right, only to shoot straight at Pickford at his near post.
McGee had to make himself big to prevent Watmore from doubling his tally from the left angle mid-way through the half, while Harrison nearly grabbed a leveller just 60 seconds later. He broke quickly, intercepting a loose ball, but he took the shot on early as Pickford advanced off his line and the ball flew just wide.
McGee used both fists to beat away Honeyman's angled drive on 76 minutes as the chances began to flow more regularly for both sides, while an intricate move involving Walker-Peters, Winks and Harrison ended with substitute Zenon Stylianides being played in behind the visitors’ defence, only for Pickford to deny him as he twisted himself into a shooting position.
Sunderland could have wrapped the game up four minutes from time when substitute Rees Greenwood wriggled free and rounded McGee but, as he pulled the ball back for Watmore with the goal gaping, Maghoma was there to make a vital intervention and the danger was cleared.
Still we chased an equaliser and one might have arrived in the final of four minutes of added time. Oduwa showed great strength to power down the left, exchanging passes with Harrison before getting to the corner of the box. With time on his side, however, he was unable to pick out a Lilywhite shirt with his cross and Sunderland cleared their lines at the expense of a corner. The resulting flag kick was then dealt with, the full-time whistle immediately following.
Spurs U21s: McGee, Walker-Peters, Ogilvie, Lesniak (McEneff 74), Veljkovic (Stylianides 74), Maghoma, Miller (McEvoy 59), Winks, Harrison, Onomah, Oduwa. Substitute (not used): Miles (GK).
Sunderland U21s: Pickford, J Robson, T Robson, Agnew, Cartwright, Beadling, Gooch, Smith, Mandron (Greenwood 85), Honeyman, Watmore. Substitutes (not used): Nelson, Casey, Blinco, Burke (GK).