Four ROPAX vessels, operated by two shipowners, provide regular ferry services between Helsinki and Stockholm. All four vessels are in satisfactory technical condition, although they are relatively old, having been in service from 1989 to 1992. Thus far, neither shipowner has published any plans to renew the vessels. However, this is likely to occur within the next ten years, given the age of the vessels and the fact that both shipowners have renewed several vessels on other ferry lines. The selection of propulsion power for new vessels is significantly influenced by EU environmental policy. While compliance is a prerequisite, the assessment of environmental sustainability is a multifaceted process encompassing a range of emissions. The objective of this study is to examine the comprehensive sustainability of prospective transitional new vessel propulsion systems. We analyze emission data from on-board emission measurements, Thetis MRV database of energy consumption of the ships, and fuel production emission factors to assess the life-cycle analysis of the VLSFO, LSMGO, gray MeOH, and LNG options. We apply the sustainability index method to furnish data for decision-making processes concerning the comparative assessment of different propulsion solutions from the dual perspectives of regulatory compliance and environmental index categories (GWP100, eutrophication, acidification, and human health particulate air potentials). We also consider the cost levels of the analyzed options, while acknowledging the difficulty in accurately predicting future costs.