Is it legal by USAT rules

Here's the actual rule:

"Excluding the bottom"

 Sweet.

If you do it correctly, and if the water is shallow enough, dolphin diving is faster than swimming.

 it's faster to swim in water than walk/run, I swim to the exit until my hand starts touching the bottom.

What is dolphin diving?  

Definatley is faster to swim than walk/run but I would say most people who do this are more for a survival tactic.  I know I did at a race earlier this year as I was slightly paniced and having a bad day and when I was trying to tread water I could feel the bottom.  I figured why not move forward instead of expending energy sitting still.

It's s technique rooted in surf lifesaving where you run until the water is about thigh-high and then dive forward with your hands in front of you. You then push forecefully off the bottom, out of the ewater and repeat.

The technique is designed to allow you swim rapidly past large waves, where you dive under a breaking wave to prevent it from impedeing your progress. But it can be used in calm, shallow water as well.

If you are talking about Cranberry Oly yesterday..lead guy stood at that point coming in as the water was so shallow. I know many folks hit lots of rocks with their hands as it was so shallow.

My family was working swim exit and was able to watch most of the swimmers.