The fly event in swimming are always a challenge. In the first place your timing have to be spot on. There is only a fraction of a second where the hands of the swimmer are out of the water and his or head is up so one can see there faces clearly.
What concern the angle it is a matter of taste. The most common angle is the one right from the front where i prefer a angel slighty from the side which prevent the photo to become to much a panoramic tipe of photo.
The photo below i took on 7 March 2009 at the SA primary Schools Aquatic Champs in Germiston Johannesburg. I used my Nikon D3 and a 600mm F4 lens.