1) The German sentence doesn't mention anything about water (das Wasser), so don't put it in the English sentence.
2) There is no need for the article in the English sentence. Acceptable answers should include "All elephants drink" and "All elephants are drinking". If you gave one of those as an answer and it was marked wrong because Duo expects you to include "the", then use the report function and choose "My answer should be correct".
Hope that helps!