Usually, my favorite watering hole while in port is still the ship; can't beat the prices. Still, I do have some favorite memories of drinks on shore.
Harry's Bar in Venice to get the original Bellini.
Caribe Hilton in San Juan to get the original Pina Colada.
The little outdoor restaurant (don't remember name) in Mykonos to get a bottle of Retsina to enjoy with Souvlaki.