This from a Watford supporters site:
As for pubs, there is the away fans only Oddfellows Arms (also known as Macs Bar) whichs address is:
14 Fearnley St
WATFORD, Hertfordshire WD18 0RD
A typical pub with a selection of lagers, ales, food and also has a BBQ in the garden when the weather permits. This is a 2 minute walk from the Chater Junior School and then another 2 minute walk to the ground, so perfect for those coming in that way. (See the map for reference as to where it is)
Other than this the usual away fans pub will be The Moons under Water (Wetherspoons) which you'll find easily, it's in the High Street and the first port of call as you come in from Watford Junction train station, a typical Wetherspoons pub with the odd ale, lots of cheap lager and cheap/cheerful food. (Depicted by the green spot on the map below)
For those who come in on the train and need a pint as soon as leaving the station, there's The Flag pub immediately on your right hand side as you come out the station, you can't miss it. It's well priced for food and drink, and has big screens/pool tables.
If you aren't so fussed about the colour wearing cheerful visitors and want to sample some real ale or a quieter pub then give the pubs near Watford Junction a try, theres a few to choose from all in close proximity.
The Escourt Arms, Escourt Tavern and the Wellington.
If you come out of Watford Junction and follow the road immediately on your left rather than up the main road (where the taxi rank is as described below) you'll find these easily.
Lastly, if you don't mind a bit more of a trek in search for good ales, then try the Southern Cross on Langley Road (WD17 4PP) or The Nascot on Stamford Rd (WD17 4QS), these are both a very short walk from Watford Junction, when you come out of the concourse, turn right and head up the road to the traffic lights/crossroads and go straight across down Langley Rd, both pubs are a 2 minute walk down there.