In total there are 8 taverns in the game and they can be found by searching the locations at the coordinates indicated below. To unlock each tavern you must walk up to the barkeeper, watch a short cutscene and then defeat the bandits. Check out the video below if you need help.
https://www.youtube.com/watch?v=eT3iT1XowZgTavern 1:
Andreas Island (coordinates 573, 720) - (0.05).
Tavern 2:
Salt Key Bank (coordinates 496, 629) - (0.25).
Tavern 3:
Crooked Island (coordinates 808, 545) - (0,58).
Tavern 4:
Arroyos (coordinates 192, 565) - (1.30).
Tavern 5:
Corozal (coordinates 37, 266) - (1.47).
Tavern 6: