This study was aimed at comparing the optimal thermodynamic performance of subcritical and supercritical organic Rankine cycle (ORC) plants for waste heat recovery from ship engines. The technical impacts of adopting a supercritical ORC scheme relative to the usual subcritical one have not been explicitly reported in the literature for heat recovery in ship engines, hence this study. The fluids R245fa, R134a, and R600a were employed for analysis due to their versatility in real systems. The ORC plants were modeled and optimized in matlab using established zero-dimensional models to satisfy the first law mass and energy balances. Results showed that introducing a recuperator would increase ORC performance. For the R600a which exhibited the best performance among the three working fluids, a net power output of 488.3 kW was obtained for the subcritical ORC without a recuperator (SYS A) and 543.7 kW for the one with a recuperator (SYS B). Furthermore, a switch to a supercritical ORC configuration increased the net power by about 29% for R134a and 10% for R600a, and increased the thermal efficiency by about 2.2 percentage points for R134a and 0.5 percentage points for R600a, referencing the supercritical configuration without a recuperator (SYS C) and SYS A.