A license issued for navigating motor boats can be accepted valid if it is clearly indicated that it can be used for navigating sailing boats as well. By all means send us a copy via email, so that we can check its validity with Croatian authorities.
If it is not indicated on a license that it is accepted for sailing boats, it can be upgraded. E.g. licenses issued in Italy can be used either for sailing boats only, or for motor boats only, or for both vessel types.