If I can... I try to get in and swim a good 200-500 yards.
It's kind of hard though... because you set up transition, that closes about 30-45 minutes before race time... and then you race. So, if you want to run, you have to run before transition closes (shoes). Even if you swim, you have to be out of the water a good 30 minutes before the race, so the warm up is all but lost.
Typically, I just stand around and wait for the race to start. That's my warmup.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"