Watford pulled up from 5-2 down to 5-5 early in the third period but then saw Southampton counter strongly and lead 9-6 with goals by Charles Roberts, Cianan Mann, Panagiotis Logothetis and Jackson Mullins.
The final period belonged to Watford. Ciaran Walshe reduced the margin to 9-7, but Watford then paid the penalty for failing to convert two man-up opportunities. Their last goal came 34 seconds from the end - by Nandor Cseh - and so, for the third time during the weekend, Watford teams suffered a single goal defeat.
In women's Division 2, Tyldesley conceded five goals to Manchester Comets before four-goal Grace Morley led a fightback. The points went to Comets in the end - by 8-7 - but Tyldesley will rue two missed penalty chances.