*Ond mae te yn ddŵr. (Remember Welsh is Verb-Subject-Object - Mae-Te(yn)-dŵr). Also I left the "yn" separate from the "Te" in this instance as it shows emphasis in the same way that you put emphasis on the "is" in English.
Diolch! I know you can rearrange the sentence for emphasis sometimes ("Owen dw i," "draig dych chi") , but I was wondering how to emphasize the verb since it already comes at the front of things. Never learn if I don't try, I suppose.