LOL - CD is right so the sooner Bede's post is hidden the better (no knowing who might read such blasphemy).
I would however add to Swimmboy's explanation of different kick requirements for water polo players.
Swimmers need the greatest speed in as perfect a straight line as possible - sometimes as sprint but also for longer distances of several pool lengths - and
there is nothing to obstruct them as in water polo. The results at the final wall touch can be determined by differences of fractions of a second and I too have seen only jammers in the men's swim events of the current Games.*
As for the girls I do not know what/if anything has been discussed on the aero-and/or hydro-dynamics of a two-piece suit.
*remember mens diving to come 2nd August - prelims 10.40 and finals 18.30 GMT (plus 1 BST)