If you already have an International Driving License then its fine. They are valid for 1 year.Not sure where you belong to in India. But for e.g in Delhi, you can get IDP by submitting the documents in below link:-http://www.delhi.gov.in/wps/wcm/connect/doit_transport/Transport/Home/Driving+Licence/International+Driving+Permit
Yes you need an IDP (International Driving Permit) for driving in Croatia.
But if one among you have an IDP then you can be the second driver holding Indian license by paying some extra euros to the rental company.
You’ll have to block your rental payments through credit card.
Hope this was helpful, Do share your experience when back.