Please don't do it. My son lost a very good friend last year doing exactly what you want to try. He didn't have a helmet on, but we've heard that the impact alone, with or without the helmet would have had the same results. I really don't want to break your spirits, but this kid severed his spinal column right there on the jump.
Please practice on a trampoline or better yet on a ramp off of a dock into the water if at all possible. And WEAR A HELMET!!!!!!