This paper presents the development of a low-order model of a marine-nuclear propulsion plant (developed using actual ship design and operational data) and its use in hardware-in-the-loop testing of a marine-nuclear plant's PLC-based steam generator water level control system (SGWLC). The model developed is based on the Nuclear Ship Savannah, the world's first operational nuclear cargo ship. The Savannah's propulsion plant utilizes a late-1950's vintage Babcock and Wilcox pressurized water reactor (PWR), however most of the components and subsystems are still represented in modern PWR designs. The plant consists of two coolant loops connecting two steam generators to the reactor vessel. The two steam generators feed a common main steam header. The main steam header provides saturated steam to the main engines and turbine generators. For reduced-order modeling, both coolant loops are combined into one hot leg - steam generator - cold leg loop with equivalent transport, thermodynamic, and heat transfer properties. Details regarding the development of a state-of-the-art version of the Savannah's steam generator water level control scheme, its deployment on a PLC platform, and details regarding the model's development and use as part of a Simulink-based HIL test bed, are presented with comparisons to available design and operational data.