What they're for!
Arguably the second most vital piece of equipment. Drag (as in the drag-coefficient, not the other sort...) is the main factor here. For male players speedo-type togs should be worn rather than shorts, no club players wear shorts, the drag is really noticeable. If you must wear shorts then it is a good idea to slit or remove the pockets since these cause a huge drag and can cause your togs to end up wrapped round your fins.

Not being female I can't offer much advice here except again to recommend avoiding anything that could cause any drag, so no little frilly skirts.