[chris (OP)]

Someone called into our Ask! The Naked Scientists show today and mentioned that some patches of the coastline of South Africa have very rough seas while, further north, the ocean is very calm.

What factors determine the roughness of the sea at a given location?

[Colin2B]

Usually to do with local geography and currents/wind.
If a current from deep water meets a shallower area eg a ridge, the water wells up and the surface becomes disturbed.
Any area where water is concentrated eg funnelled, or if a current passes a headland it’s flow will become concentrated ie higher speed and the current flowing out into the passing current will cause disturbed water. If the headland extends out underwater then this will increase the effect because of upwelling.
Any area where 2 currents meet will also result in disturbed water, particularly if there is a significant difference in direction.
You can also get an interesting effect if there is an offshore mound just under the surface. This will cause incoming waves to be refracted so that they converge on the inland side giving a very distubed sea with no visible cause, it’s only when you look at the wave pattern that you can see what is happening.
All the above will be worse if the prevailing wind is in opposition to the current - you can get some very short, steep waves.
There are some areas around UK where it can be like being in a washing machine.