"Fetele o să umple paharele cu apă."

Translation: The girls will fill the glasses of water.

[trashjedi]

Should the correct English be "the girls will fill the glasses with water"?

[danubissim]

Yes, it should be like you said.

[Renardo_11]

It is an accepted answer (now), although not the proposed one.

[poftim]

Yes. In Romanian "un pahar cu apă" and un pahar de apă" both mean a glass of water, but obviously that doesn't work when you're filling a glass.