It is hands down one of the most bad ass things to say to a newbie during their first class. I have heard it credited to Rickson Gracie, Renzo Gracie, Royce Gracie, and even Rolls Gracie.
Closer to it's originator (but still incorrect) is Jean Jacques Machado, and Rigan Machado.
Well, I found the source.. I was reading though old Black Belt Magazine issues in Google Books and came across this...
|Black Belt Magazine, November 1994, Page 27|
The quote is attributed to Carlos Machado and appears on page 27 of the November 1994 issue of Black Belt Magazine in an article about Royce Gracie's take-downs
The full quote is
"After the clinch, it doesn't matter what happens, one way or another, we're going to hit the ground, and we'll be in my world. The ground is my ocean, I'm the shark, and most people don't even know how to swim."
Cool quote... it's even cooler that it is attributed to the right person.