The 18-year-old Hungarian striker is highly-rated at the Serie A club and is being chased by a host of clubs across the continent.
Otto Novothny, the player's father, believes staying at Napoli is the most likely scenario but a switch to work with Hornets boss - and former Napoli star - Gianfranco Zola cannot be ruled out.
"It seems that Soma will play in the under-age team at Napoli but there is a chance that he will go to another team on loan - maybe in Serie A or Serie B," he told Hungarian daily Kisalfold.
"But it is also possible that my son will go to another country. Hoffenheim are interested in him as well as FC Zurich and Dnipro (Dnipropetrovsk).
"But his former coach at Napoli works as Zola's assistant at Watford and he'd like Soma to sign there on loan."