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This paper introduces the concept of additively manufactured (AM) water filled parts (WFPs). By combining the energy absorbing properties of water with polymers it is possible to significantly improve time to ignition of AM parts with open internal structures. Theory relating the flame temperature to the maximum wall temperature of WFPs is developed. A range of water-polymer configurations are presented as a basis for WFP designs. Three separate thermal experiments were conducted to test different aspects of the WFPs. The time to ignition for cone calorimetry samples was extended 794% over plain photopolymers. Case studies were used to demonstrate the effectiveness of WFPs with complex shapes. The results of thermo-fluid finite element simulations showed good agreement with experimental observations and provide a useful tool for the evaluation and optimisation of WFP designs. The fire resistance of thin walled structures were found to be significantly improved by adding water. The water filling strategy was found to be more effective than adding intumescent coatings. Finally further work and recommendations are discussed.