The wave energy industry has grown considerably over the past two decades, developing many different technologies, but still has not achieved economic viability. Economic performance can be significantly assisted through the optimisation of the device intelligence, implemented as a control algorithm, including precise hydrodynamic models able to reproduce accurately real device motions. Wave energy converters (WECs) are designed to maximize power absorption with large motions, suggesting significant non-linear behaviour, and so precise non-linear models are imperative. Although similar modelling philosophies could be applied to the wide variety of WECs, the dominant physical phenomena vary from one device to another. As a consequence, different non-linear effects should be considered in each system, but only those being relevant should be chosen to avoid extra computational costs and potential numerical problems. This paper studies different nonlinear hydrodynamic effects and their influence on the various types of WECs, and suggests a critical classification of different methods to articulate nonlinear effects.