I would only worry about giving water when you are ready to introduce sippy cups. If you're ready now, then go ahead. I would put enough water in the cup so that they can comfortably drink from it (don't have to stare at the ceiling to tilt it up enough), but not so heavy they can't lift it.
Other than that, it shouldn't affect the baby's breastfeeding if you are only doing the cup at the table - no giving them the cup as a toy, or to take around the house. The cup should be for mealtime, not playtime.