This course should teach a bit slang Hindi. This is common. The synonym of 'paani' is 'jal' or 'neer'.
I guess the course's aim is to teach words that are most relevant in everyday speech. Barely anybody uses 'जल' or 'नीर', and teaching those words would probably give new learners too much to remember.
Jal isn't used much at all unless you're watching 'Vivaah' and that word comes up 50 times