First of all, go for this achievement in training mode, you can keep restarting until you get a perfect start. I unlocked the achievement in this mode and definitely recommend for practice, as well as keeping rhythm for the achievement attempt
In order to get a perfect start I recommend lightly holding the Left Trigger until jumping off the platform at the same time Go appears on the screen. Keep aiming for a perfect start and eventually you will master the start.
In order to gain maximum speed I recommend "waggling" the analogue sticks. This will involve rotating the Left Thumbstick clockwise and the Right Thumbstick anti-clockwise. Personally, I found my thumbs worked best for this but I know my friend used his index fingers on each thumbstick and seemed to be getting really good times, kind of depends on personally what you find practical and comfortable. Skipping the Thumbsticks will result in a loss of speed, therefore try to make sure the thumbstick is constantly touching the parameters. Make sure on the turnover you score perfect by hitting the Left Trigger as soon as the swimmers foot hits the swimming pool tiles.
I found that my thumbs would tire over time, as soon as I picked up on this I would immediately pause, let my thumbs rest until I personally felt happy I could play a little longer. If you just keep doing this method you will maintain the speed and composure needed to get the achievement, I managed to get .20 below the time required. Hope this helps.