This paper investigates the preferences for institutional mechanisms for improved water supply services across different ethnic communities in slums of Kolkata. The Muslim community prefers privatization of water supply as against paid public supply. The backward caste community prefers both paid public delivery and privatization. Residents of non-notified (NN) slums prefer paid public delivery as against privatization. Access to accountability mechanisms for water supply is lower for residents of Muslim dominated regions and NN areas. This is reflected by household perception about awareness of councilors regarding water supply conditions in the slums. The choice of alternative institution depends on the degree of risk of exclusion due to lack of access to accountability mechanisms. Notification of NN slums, higher revenue autonomy and capacity of local bodies, and innovations in scale neutral technologies may improve access to water supply by marginalized communities in slums.