This paper presents a modularity analysis for the design of complex products, based on domain theory, applied to the case study of the design of an electro-solar vessel to be implemented in South American rivers. The proposed approach allows obtaining graphical information from the functional analysis tools, about the interaction between functional modules or organs of the system in terms of material, energy, or information flows; useful to analyze the functional architecture, the types of modularity and the interactions in the system. This information is complemented with the approach to the physical architecture of the system identifying interferences and restrictions. This information is intended to be useful in the development of methods or methodologies for the definition of the assembly sequence.