Most important, Get close; most like to see face shots and the water looking like ice. Early shots (before they breathe) are real good also; the swimmer appears to be shrink wrapped in cellophane. Don’t worry too much about getting all arms or hands completely in the frame, if you have the face tack sharp the image works.
Don’t use an Extender, to slow (AF). Cuts down the keeper rate
I’ve spent a few years shooting only aquatics per say, and have built a nice library of images. I shoot with 20D and 1dmIIn body and my prim lens is 300mm f/2.8L IS or 70-200 IS. I shoot any where am allowed to be. Most of the time if it’s within my range of filling 40% of my frame I start shooting no matter where I am, standing or kneeling.
Swimming shots are best during long cores, it give you a better angle at getting more swimmers in one frame.
freestyle,
I watch to see witch side the swimmier is breathing from on there flip turn to insure face time and then shoot standing at a 45 degree to the swimmer with my camera at a 20 degree in my hand to give the appearance their swimming up hill, And shoot low if you want other swimmers in the frame.
Breast
has to be the easiest of all, they have to come up every stroke, I prefer a side shot but right in line with the swimmer looks good also, low or high both work. And shooting from the side will give you more keepers as far as auto focus.
Fly
works best from the side if you want a tight crop and other swimmers in the shot,But a straight on face shot works well.
Back
is the hardest to get, most of the time the swimmer is splashing water over their face, so I shoot it from the side mostly. Back is great if you’re standing in lane 1 and shooting lane 1, then shoot them streamlining under the water as you walk the deck just keep shooting and shooting.
I shoot manual because Aperture or Shutter prioritys gives you very inconstant exposures due to the light changing with the plashes.
One trade secrete that I will share with you, but don’t tell any one else; I shoot many daytime images with my 580EX, CP-E3 battery pack in highsink mode and 1/3 to 2/3 stop Under, this makes the water look more like ice and stand out of the images.
But don't flash the Starts, very bad.
Samples
http://www.tesoroaquatics.com/proposal/index.html
Averages
Out Side
Al-Servo
1600th
Iso 400
Spot metering on the face
Burst of 3 to 4 frames
Indoors
Al-Servo
F2.8
320th
Iso 1600
Spot metering on the face
Burst of 3 to 4 frames