A nonlinear hybrid wave model (HWM) is developed. It uses the conventional mode-coupling method (MCM) and the phase modulation method (PMM) to address the nonlinear interactions between free-wave components in an ocean wave field. The PMM is a complementary solution to the divergence of the MCM because the MCM may not converge if the wave lengths of the two interacting wave components are quite different. The HWM divides a wave spectrum into several wave bands and employs the MCM to formulate the wave-wave interactions of close frequency wave components and the PMM to formulate interactions of wave components that are relatively far apart in the frequency domain. Wave kinematics under irregular ocean waves is predicted by HWM. The nonlinear effects on the kinematics are significant above the still water level (SWL). For non-narrow spectra the prediction of wave kinematics by the MCM may be divergent. The HWM is applied to study the wave nonlinearity and surface intermittency effects on a single degree of freedom (SDOF) structure. It is concluded that the surface intermittency effects and wave nonlinearity are both important under relatively strong nonlinear waves when linear wave theory and modified linear theory may considerably underestimate the wave forces. The sensitivity of wave forces to the cutoff frequency, that was found by Hu et al, (1995), is caused by the divergence of the MCM for broad-band and relatively steep spectra. Based on the analysis and simulation, vital conclusions of the nonlinear wave effects on wave loads of SDOF structures are given. Important conclusion of the computation cutoff frequency in spectral design practice is also given in this thesis.