The St Anne’s attacker came off the bench with just 16 minutes to go and blasted 2-1 to leave them in pole position in Group D.
Louth were in a strong position with 20 minutes to go as they led by three points but a 1-4 scoring burst put Jim Gavin’s side into the lead and Watson added his second goal a minute from the end of normal time to seal it.
Watson’s first goal was a neat curled effort into the side of the net and his second was a clever toe poke past Shane McCoy.
It was a night of firsts as Dublin also picked up their first ever black card, defender Darragh Nelson walking to the line for an off-the-ball drag down in the second-half.
After a dull first-half that yielded just four points, Louth led 0-3 to 0-1 thanks to points from Brian White, Derek Crilly and Liam Shevlin.
Dublin’s first sanction under the new rules finally arrived eight minutes into the second-half when Nelson hauled Stephen Campbell to the ground.
It didn’t get any better in scoring terms either for Dublin with White’s second and third points of the game taking Louth 0-5 to 0-2 clear.
But the hosts hit back with 1-4 without reply that included Watson’s first goal and three points in-a-row from Paul Hudson.
And though Louth pulled a couple of late points back, Watson’s second goal in the 69th minute sealed victory.
Scorers for Dublin: D Watson 2-1, P Hudson 0-3 (0-1f), J Whelan 0-1, S Carty 0-1
Scorers for Louth: B White 0-3 (0-1f), D Crilly 0-2 (0-1 45), L Shevlin 0-1, D Byrne 0-1, S Campbell 0-1
DUBLIN: S Currie; D Nelson, S George, M Fitzsimons; C Guckian, B Cullen, K Nolan; D O’Mahony, S Carty; C Walsh, P McMahon, J Whelan; P Hudson, K McManamon, G Sweeney.
Subs: J Cooper for Nolan h/t, E Fanning for Walsh 48, N McGovern for Guckian 43, L Fletcher for Nelson black card 43, D Watson for Sweeney 54, C Mullins for Cullen 65.
LOUTH: S McCoy; P Rath, D Finnegan, K McLoughlin; D O’Hanlon, D Crilly, L Shevlin; P Keenan, R Brodigan; D Byrne, G Connolly, B Duffy; K Rogers, B White, S Campbell.
Subs: D Maguire for Finnegan h/t, C Doyle for Rogers 43, P Sheelan for Brodigan 43, D O’Connell for White 59.
Referee: F Kelly (Longford).