"I only drink water."
Translation:Piji jen vodu.
piju is certainly accepted
piji is older and now formal, piju is progressive, less formal and more common in everyday speech.
This word order means something else than Piji jen vodu.
Piji jen vodu. means that water is the only thing I drink.
Jen piju vodu. would be used "Co to děláš?" "What are you doing?" "Jen piju vodu." "I am just drinking water."
I do not think it is a good translation here with the past simple tense.