Add grog kegs that you can pick up and walk around with. Interacting with them while holding a tankard refills the tankard.
These grog kegs can sometimes be dug up on islands, be found in shipwrecks and are sometimes featured in voyages and quests.
There could be a normal grog keg which sells for an average amount, and then there could be a special grog keg, let's call it the "Pirate Legend's Grog Keg", which fills your tankard with grog that hits you 2-3 times as hard as normal grog and the keg itself also sells for a lot more. Of course other variants such as Ashen and Coral grog kegs could also be made.
EDIT: There is already something like this in the game - the Chest of a Thousand Grogs - which is a cursed item and has 1000 refills available for your tankard. So think of my suggestion as adding non-cursed variants of it which have different rarities, different methods to obtain them, different sell values, different amounts of refills and different special effects.
