Coffee is generally accepted as ok. I do a 16/8 almost every day and I drink coffee while I am fasting. Water is fine also and you should definitely hydrate when thirsty. I have been tossing around the idea of avoiding coffee for a week while I am fasting just to see what would happen and if my results would change but I have not gotten there yet.
Liquids with no calories or low calories will not derail an IF.