Fraizer Campbell had a great chance to put Hull ahead within ten minutes as the Manchester United loanee latched on to a poor Watford header on the edge of the area but the Hull frontman could only fire the ball straight at keeper Richard Lee.
Watford took the lead on 12 minutes after clever play inside the Hull box created space for Henderson who neatly slipped the ball right footed past Boaz Myhill from ten yards.
Hull levelled the scores just before half-time when Watford failed to clear a header up field from Andy Dawson and the ball came to Garcia who managed to loop his header over the stranded Lee and Barmby raced in to nod the ball into the back of the net.
Substitute Tamas Priskin nearly put Watford ahead seconds after coming on in the second half, but his right-footed strike from ten yards was parried away by Myhill.
Hull went in front on 70 minutes when Sam Ricketts raced down the right and crossed towards substitute Folan who neatly headed the ball past Lee from five yards to send the home crowd into early celebrations.
Substitute Craig Fagan went close from 30 yards towards the end of the game, as his fierce long-range effort whistled just over the bar.
Watford came close to pulling one back late on as Jobi McAnuff swung in a corner towards Henderson whose header from six yards had to be cleared off the line by Bryan Hughes.
Hull grabbed a third on 88 minutes when Garcia cleverly slipped through the Watford defence and slotted the ball home past Lee.
The Tigers then added a fourth in stoppage time when substitute Doyle fired in a shot from ten yards which deflected off a Watford defender and floated into the back of the net.