We use a straw cup. To start with it was a matter of exploring the straw and then became a yay I can drink for this. I personally use a Stanley dup so she watches me drink from a straw and wanted to copy
He happy drink out of cup and beaker if got milk in as tried that but not really water
Have you tried a straw cup? It's most likely just a taste difference issue. I would just keep offering, or you could even water down breast milk gradually until its just water, kinda the same process when transitioning from BM/formula to whole milk!