"We will drink water tomorrow."
Translation:Byddwn ni'n yfed dŵr yfory.
To the original English - 'We will drink water tomorrow', your suggestion is a better one than the one above, since it is talking about one action in the future, here are the possible translations:-
'We will drink water tomorrow' - Yfwn ni dŵr yfory/Mi yfwn ni dŵr yfory/Mi wnawn ni yfed dŵr yfory/Gwnawn ni yfed dŵr yfory.
However the English translation above doesn't give the proper meaning of 'Byddwn ni....' this is referring to a continuous action in the future.
The better English translation is:- 'We will be drinking water tomorrow'.
I've altered the English translation of this exercise but I've also added your suggestion since it would be likely to be used in spoken Welsh.
Thanks for the comment.