Charlie McDonald grabbed his first goal for Southend and created another as they knocked an under-strength Watford side out of the Carling Cup.
Tamas Priskin and Anthony McNamee both went close for the Hornets early on but McDonald put the home side ahead with a fine shot from the edge of the box.
Matt Jackson was denied a leveller by a decent save by Daryl Flahavan.
But the Shrimpers sealed the win when McDonald played a neat pass into Matt Harrold who finished from close range.
Southend: Flahavan, Francis, Richards, Barrett, Hunt, Bailey, McCormack (Lokando 90), Moussa (Hooper 90), Black, Harrold, MacDonald.
Subs Not Used: Clarke, Collis, Gilbert.
Booked: Barrett, Francis, McCormack.
Goals: MacDonald 20, Harrold 55.
Watford: Lee, Mariappa, Jackson, Avinel, Parkes (O'Toole 71), McNamee, Bangura, Rinaldi, Campana, Priskin, Hoskins.
Subs Not Used: Loach, Forbes, Osborne, Liam Henderson.
Booked: Avinel, Bangura, Rinaldi.
Ref: Jarnail Singh (Middlesex).