Offshore fish farms are usually set in open ocean areas along with the extreme wave-current loadings from high-energy sea states resulting in increased fatigue and unpredictable structural damages, thus the safety issues in the extreme sea states have attracted widespread attention in the aquacultural industries. The flow fields around and the hydrodynamic loadings on the nets, as well as the stability of the structure in the non-linear waves and currents, are the critical issues related to the safe operations of aquacultures. An advanced understanding of waves and currents through aquaculture structures is required to quantify the hydrodynamics loads as well as dynamic responses and, thereby, optimize future aquaculture structures designs. With the improvement of computational resources, the numerical simulation techniques based on the fluid structure interaction has become a promising strategy to study the hydrodynamics of offshore fish farms. It includes a CFD fluid solver for the three-dimensional numerical wave tank, a novel continuous direct forcing approach for coupling three-dimensional moored floating bodies with a net solver. To couple with NWTs, a Lagrangian approach for nets is implemented in an Eulerian fluid model, and hydrodynamic loads on nets are determined using screen force models. The integrated numerical framework has been implemented in the REEF3D open-source hydrodynamics software. The results are primarily illustrated in waves, the dominance of currents was however overlooked. In this study, high-fidelity and efficient FSI simulations using the aforementioned REEF3D are utilized to study the hydrodynamics of a semi-submersible offshore fish farm in waves and currents. The generation of inputs conditions and the wave absorption are dealt with the relaxation method and the active wave absorption methods, and the accuracies of NWT are validated with the published data. The flow fields around the semi-submersible offshore fish farm in pure currents are initially revealed. The impacts of Reynolds numbers and bio-fouling are considered, of which the latter are modelled through net solidities. Secondly, the regular and irregular waves combined with currents are included to examine response motions, wave loadings, mooring line tensions and offset distances. This study has provided the scientific references for the designs and optimizations of semi-submersible offshore fish farms.