Link goes to a list of regular cocktails a bartender could make n/a versions of, like a mojito, mule, piña colada, etc. You also can ask what fruit juices they have and get one with club soda (pineapple, grapefruit, cranberry, orange should be there).
If it's really busy and loud, and hard to communicate, club soda+lime or lemon should be easily understood and there's a decent chance they won't charge you (have cash to tip anyway!). Otherwise, don't be afraid to ask the bartender, too.
The bar might have n/a beer or spirits. Check their website and see if there's a beverage menu you can look at before you go.