GSManiac:

 

The stupid three second countdown timer makes it difficult to start the timer exactly as you pass the start line.

 

Just on point 3, you can tap the screen to bypass the 3 second countdown and start the workout immediately.

 

In practice this doesn't really help because you have to start the activity approximately two seconds before crossing the start line to ensure that you have the opportunity to interrupt the three second countdown as you cross the start line. Anyone who does competitive events regularly will tell you that this is awkward.

It's not ideal, but I tend to follow this process:

• About a minute before the start horn I start the activity then immediately pause it by depressing the crown + side button.
• I then engage the water lock mode.
• I use the crown + side button to unpause as I'm crossing the start line. 
• As I cross the finish line I use the crown + side button again.