The emerging non-regulated incursion of urban life into the water realm in waterfront cities could represent a threat to the ecosystem if not adequately led and controlled by an interdisciplinary, sustainable, and data-driven approach. The paper provides a proof of concept for a computational design support system to advance performance-driven reasoning in loating building design. The proposed user interface design platform emphasises a quality score for several design aspects and the logical interaction between different potential choices to identify optimal decisions in the presence of tradeoffs between two or more conflicting requirements. The design support system is based on a performance-based design-support framework for floating buildings developed in previous studies. The performance requirements are converted into quantitative, measurable indicators and introduced in a Grasshopper script to integrate and control the design workflow. A custom user interface to control Grasshopper definitions is designed using the Grasshopper plugin Human UI. The tool enables designers to interactively explore and evaluate different design options according to their design objectives. Practitioners have tested the tool by applying it to two existing projects. Its practical implementation provides valuable feedback before building a full-scope system and proves its feasibility and usability regardless of location. Despite being conceived for the design phase of new structures, the paper demonstrates its twofold purpose, as it can also be used for evaluating existing projects or built structures, hence used as a certification system. The potential for upscaling the design support tool to urban scales opens up new possibilities for sustainable and resilient urban development in waterfront cities.