A flexible, modular, submersible fish cage is an attractive design concept for offshore fish farming due to its cost-effectiveness in construction and survivability during extreme weather events when submerged at an appropriate water depth. However, under some intensive circumstances, the structure would experience extreme weather conditions at the water surface, which leads to potential failure. This paper investigates the hydroelastic behaviour of a simplified design of a newly proposed modular fish cage, named the SeaFisher, under various wave conditions, including waves in severe weather conditions at sea level. The software AquaSim is employed to model the cage frame, nets, and mooring line system and perform the hydroelastic analysis to assess the strength, stiffness, and stability of the structure in floating conditions. The study highlights the need for an optimal design to prevent large deformations and potential failure of the fish cage. As wave height and period increase, the axial compressive forces within the structural members also rise.