The 2.05m 17-year-old beat another youngster, 15-year-old Thomas Hartono, 6-4, 3-6, 6-3 at the Edgar Centre.
Watt used his powerful serve to great effect, being broken just once — in the second set — when he double faulted twice.
Indeed, that was enough for Hartono to nab the set, but Watt ensured he did not slip up again.
With the chances of Hartono breaking back looking slim, it always felt as though Watt was in control in each set after gaining those breaks.
Watt acknowledged the serve was key and said the surface made it more dominant than normal.
"Usually it is my go-to weapon, but on this surface it really kicks on and obviously it dominated the battle."
Hartono had been something of a surprise finalist, having entered as an unseeded player.
That meant he played four games, one more than Watt, which included three three-set wins on Saturday.
He put in a tidy display, holding his serve on all but two occasions and doing well to get the ball over Watt on several occasions.
Watt said he was happy to get the win and acknowledged Hartono was a strong opponent.
"It was a tough battle, credit to Thomas for playing four matches to three sets and giving me a good run around."
In the women’s final, Megan Rogers overcame a sore shoulder to beat top seed Tessa McCann 7-5, 3-6, 6-2.
McCann led 4-2 in the first set before Rogers found another gear to level the set, then took a 6-5 lead and proceeded to serve out the set.
However, McCann fought back in the second set and broke Rogers twice to shoot out to a 4-1 lead. She began moving better and forced Rogers into several errors, using consistent power and depth.
When she emerged from the second set as the victor momentum appeared to have swung, McCann seeming to have the upper hand.
However, Rogers showed her power in the final set, racing out to a 3-0 lead after a strong forehand enabled her to break McCann early.
McCann fought back to win a game, but Rogers broke again and then held serve to take it to 5-1. Again McCann took a game, before Rogers claimed the win.In the men’s doubles final, Ryan Eggers and Paddy Ou beat Jeff Elliotte and Carlos Reid 6-3, 6-2.
Victoria Hartland and Megan Rogers claimed the women’s doubles title with a 6-2, 6-4 win over Aimee Brown and Tessa McCann in the final.
