Onshore wind turbine technology is moving offshore, and the offshore wind industry tends to use larger turbines than those used over land. This calls for an improved understanding of the marine boundary layer. The standards used in the design of offshore wind turbines, particularly the rotor–nacelle assembly, are similar to those used for onshore wind turbines. As a result, simplifications regarding the marine boundary layer are made. Atmospheric stability considerations and wave effects, including the dynamic sea surface roughness, are two major factors affecting flow over sea versus land. Neutral stratification and a flat, smooth sea surface are routinely used as assumptions in wind energy calculations. Newly published literature in the field reveals that the assumption of a neutral stratification is not necessarily a conservative approach. Design tests based on neutral stratification give the lowest fatigue damage on the rotors. Turbulence, heat exchange and momentum transfer depend on the sea state, but this is usually ignored, and the sea surface is thought of as level and smooth. Field experiments and numerical simulations show that during swell conditions, the wind profile will no longer exhibit a logarithmic shape, and the surface drag relies on the sea state. Stratification and sea state are parameters that can be accounted for, and they should therefore be considered in design calculations, energy assessments and power output predictions. Copyright © 2012 John Wiley & Sons, Ltd.