With Mauricio Pochettino's men looking to reel in leaders Chelsea, top goalscorer Harry Kane was back on the bench less than a month after suffering an ankle injury.
But once again Spurs proved they do not need to rely on Kane's goals as Son struck either side of half-time to wrap up a convincing 4-0 win after quality finishes from Dele Alli and Eric Dier had set them on their way.
Tottenham, who also hit the crossbar three times, were never likely to suffer defeat against a Watford side plagued by injuries as Walter Mazzarri shuffled his pack with Troy Deeney only on the bench due to fatigue.
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Tottenham XI: Lloris; Trippier, Alderweireld, Vertonghen, Davies; Dier, Dembele; Son, Alli, Eriksen; Janssen
Subs: Walker, Kane, Wanyama, Nkoudou, Sissoko, Wimmer, Pau Lopez
Watford XI: Gomes, Janmaat, Cathcart, Mariappa, Holebas, Amrabat, Doucoure, Cleverley, Niang, Okaka, Success
Subs: Deeney, Zuniga, Kabasele, Capoue, Pantilimon, Mason, Eleftheriou
Referee: Anthony Taylor (Cheshire)