Taken from IGN to save you the Google Search:
The tavern locations and their coordinates are as follows:
#1 - Andreas Island (573,720)
#2 - Salt Key Bank (496, 629)
#3 - Crooked Island (808, 545)
#4 - Arroyos (192, 565)
#5 - Corozal (37, 266)
#6 - Grand Cayman (392, 325)
#7 - Kingston (623, 172)
#8 - Ile à Vache (839, 137)
Now, embellishing on that:
To unlock each tavern, you talk to the bartender, which initiates a fight with 4 hooligans. You have to beat up the hooligans without weapons to unlock the bar. I recommend waiting on the opportunity to counter each one, then press square immediately after countering, which knocks out your opponent.