This study assesses a poly-generation system, driven by low-grade industrial waste steam, that is electrically and hydraulically self-sufficient — its hydrogen electrolyzer draws power and feedwater entirely from upstream subsystems within the cascade rather than from grid electricity or external freshwater supply — while depending on a continuous waste-steam stream, seawater feed, and auxiliary cooling infrastructure as its external thermal and material boundary conditions. The system features a four-stage thermodynamic cascade comprising an Organic Rankine Cycle (ORC), a Thermoelectric Generator (TEG), a Humidification-Dehumidification (HDH) desalination unit, and a Proton Exchange Membrane (PEM) electrolyzer. The study was carried out for the ORC working fluids R245fa, Cyclopentane, and n-Pentane, at different waste steam temperatures and qualities. A rigorous mathematical model was developed in MATLAB, with each subsystem independently validated against dedicated experimental datasets; full four-stage cascade validation was not attempted because no equivalent integrated experimental system exists in the literature. The results show that using R245fa resulted in significant operational instability, with a 16% power drop. Cyclopentane eliminates the transition penalty and ensures a smooth, monotonic rise in power across the full temperature range. Under the highest steam operating conditions (200 °C, saturated vapor), the system utilizing Cyclopentane delivers a maximum net power of 998.48 kW, a peak hydrogen production rate of 22.12 kg/h, and an overall system efficiency of 12.82%, representing a 25% improvement over the R245fa baseline. A complementary exergy analysis reveals that Cyclopentane also achieves the highest overall second-law efficiency (18.20%). Furthermore, component-level assessments identify the HDH unit as the primary thermodynamic bottleneck with the lowest exergetic efficiency (0.19%). In contrast, the PEM electrolyzer achieves the highest (>73%), confirming the rational allocation of high-quality electrical exergy toward hydrogen production. Simultaneously, the HDH unit secures a substantial freshwater surplus of 1948 L/h beyond the electrolyzer's consumption. The study highlights a strategic trade-off: while saturated steam maximizes electrical output, lower steam quality (0.8) maximizes the Energy Utilization Factor of a peak of 39.23%. Techno-economic and environmental assessments confirm the system's viability; utilizing Cyclopentane at 200 °C minimizes the Levelized Cost of Hydrogen (LCOH) to an optimal 1.783 USD/kg, yielding a 20-year Net Present Value (NPV) of 4967.5 kUSD with a rapid payback period (PBP) of 2.49 yr, while mitigating 3775.8 tons of CO2 annually.