"Keoki drinks water."
Translation:Inu ʻo Keoki i ka wai.
I believe you have to introduce the object with "i," so it would be "i wai." I also think the "ka" is required. Hawaiian seems to use "ka" a lot more than English uses "the." So it would have to be "i ka wai."
What I donʻt know is if you can swap the order of the subject and object. Can someone more advanced please tell us if "Inu i ka wai ʻo Keoki" would work?