This paper presents a novel design-driven methodology for fabricating marine sandwich structures using additive manufacturing (AM). The approach leverages the geometric freedom enabled by Fused Filament Fabrication (FFF) to produce complex lattice cores from PA12-GF15, a fibreglass-reinforced nylon, directly integrated into sandwich panels with laminated glass fibre skins. A hull portion of the Chichester Scow dinghy was selected as a case study to demonstrate the method’s feasibility. The core geometry was generated through volumetric lattice modelling in Blender, enabling full customisation of the internal structure to match the outer hull surface. The resulting sandwich panel was produced entirely in-house and compared against traditional fibreglass laminates and aluminium-honeycomb-core panels. The results show that the 3D-printed sandwich structure achieves an approximate 58,7% weight reduction compared to solid fibreglass construction, while also offering improved manufacturing flexibility and potential for performance tuning through localised lattice control. This study highlights the potential of AM in marine engineering and lays the groundwork for future research on structural behaviour and large-scale implementation.