An experimental investigation on the coupled dynamics of an offshore tunnel-rig system from the element launching to its near-seabed immersion in irregular waves is conducted, where the motions of tunnel/rigs and the tensile forces of the suspension lines are evaluated in a synchronous manner for a wide range of irregular waves. The effect of immersion depth on the coupled dynamics of the tunnel-rigs system, especially the relationship between the relative motion and suspension line tensions are experimentally examined. The system coupling mechanism is further addressed using the modal analysis and response spectra analysis. The results show that the natural frequencies of the coupled system are primarily affected by the varying immersion depth. The increasing immersion depth leads to a larger in-phase roll period of the system and a greater roll resonance amplitude. For a given immersion depth, a strong dependence of the suspension line tensions upon the relative motions is established.