The shipping industry's need to reduce greenhouse gas emissions has driven the exploration of technological alternatives, including adopting low or zero-carbon fuels. While gas turbines have proven effective in specialized segments such as naval vessels and fast ferries, their use in merchant and passenger ships is limited due to lower efficiency compared to traditional marine diesel engines. However, the drive for decarbonization is changing this landscape. Gas turbines can efficiently utilize alternative fuels like bio or synthetic natural gas, methanol, and hydrogen. The successful adoption of gas turbines in commercial vessels hinges on two factors: significantly improving efficiency via combined cycle steam turbine power generation (COGES) and the competitive use of alternative fuels. This study examines a marine gas turbine combined cycle concept, aiming to match or surpass the efficiency of diesel engines while reducing CO2 emissions. Using advanced process modelling tools, the study develops and simulates a COGES system, proposing a novel heat transfer model for part-load operations. Optimal operating parameters are determined through parametric analysis across a range of loads using various alternative fuels. A case study on a modern cruise ship combines COGES with diesel engines, optimizing power management to maintain high efficiency across load variations. Results demonstrate that the integrated COGES-diesel system can achieve comparable efficiency and substantial CO2 emissions reductions using low or zero-carbon fuels, providing a viable pathway for the decarbonization of maritime transport.