Water waves moving from shallow to deeper water
Light waves will speed up or slow down when they enter or exit a material of a different optical density, which is the refractive index of the material. If the refractive index of the material is higher than the refractive index of air (which has the value of 1.0), then light will travel slower in the material.
However, water waves are a bit different because these waves are mechanical waves - it's the oscillations of actual water molecules which cause the wave to move. Water waves are actually called 'Rayleigh waves'. For GCSE purposes we treat them as being transverse waves, but they're actually more complex than that - here's an animation:
If you look at a particle closely, you'll see that it oscillates both perpendicular and parallel to the direction of the wave motion. So a Rayleigh wave is a mixture of a longitudinal and a transverse wave!
The depth of water affects the speed of these waves directly without having anything to do with the density of the water. The deeper the water, the faster the waves travel, and so waves will refract (change direction) when they enter deeper or shallower water at an angle.
GCSE Keywords: water waves, shallow, deep, wave speed, wavelength, direction