Gerstner waves
A wave model in which each water particle moves in a circle, giving sharp crests and broad troughs instead of a smooth sine curve.
The model is named after Franz Josef Gerstner, who described the motion as far back as 1802. Unlike a height-field wave, the points are displaced both vertically and horizontally, so the crest bunches up and sharpens — just like real ocean swells. A handful of waves with different directions, wavelengths and amplitudes are summed in the vertex shader into a convincing surface.
The technique was popularised for real time in GPU Gems (Finch, 2004). It is cheap and controllable but scales poorly to the open ocean; for that you switch to an FFT-based method (Tessendorf) that computes thousands of waves at once. Normals are derived analytically so the lighting holds up.