The sides were level at 2-2 after the first period but Watford dominated the second, scoring six goals - by Andy Holt, Ciaran Walshe, Ilija Mustur, Danny Davis, James Warman and Drazen Kujacic - with only one against. Kuracic finished with three goals.
Croydon Amphibians outplayed Birmingham 20-10, including goals by Csaba Rekasi (5), Jon Cannon (3), Alfonso Tucay (3) and Kevin Steele (3).
Welsh Wanderers lost for the first time - 9-8 to Southampton, whose winner three minutes from the end was claimed by Anand Vaithilingam.
Watford won again - 19-11 against Ealing - with Ilija Mustur again on target with seven goals.