The recent European regulations on emissions from heavy duty vehicles (Euro VI) along with the enforcement of ECA regulations have represented an additional challenge for the sustainability of the motorways of the sea. The main aim of this paper is to identify the optimal sizing and the most adequate propulsion plant for a fleet of feeder vessels that, by operating under motorways of the sea conditions, is able to articulate competitive intermodal chains versus the road for the door-to-door transport by ensuring the sustainability of the intermodality in the current normative framework. Thus, a mathematical model is developed to evaluate, aside from the total costs and the time invested in the transport, the environmental costs of the unimodal transport and of intermodal chains with different sizing and technologies for the vessels. The resolution of this multiobjective model was carried out with an NSGA-II algorithm in an application to a transport network between Spain and France. This application concluded that fast and small vessels with LNG propulsion plants are the most convenient to maximize the competitiveness advantage against the road alternative. Likewise, the analysis of the environmental performance of both transport systems in the application case from 2010 to 2015 shows an unfavourable environmental evolution for the intermodality.