Abstract: To investigate the seismic nonlinear response of seafloor sites, this study analyzes S-net seafloor strong-motion records from the M_w7.4 off-Fukushima earthquake on March 16, 2022. Horizontal-to-vertical spectral ratios (HVSR) are compared between strong motions (PGA≥100 cm/s²) and weak motions (20 cm/s² < PGA < 100 cm/s²). Three nonlinear site parameters (DNL, ADNL, PNL) are examined to evaluate nonlinearity at 148 ocean bottom stations (OBSs), and regression analysis is conducted on their relations with PGA and PGV for buried and unburied OBSs. Results show that when horizontal PGA exceeded 100 cm/s², some OBSs exhibit clear nonlinear features, including dominant frequency shift to lower frequencies and spectral ratio reduction at high frequencies. All three parameters correlate well with PGA and PGV; PNL is more suitable for nonlinearity identification at S-net sites, while DNL and ADNL thresholds for offshore sites require reassessment. The setting condition of OBSs significantly affects nonlinearity assessment, suggesting buried and non-buried OBSs should be analyzed separately.