Tube/Flash advice...?

My 3rd water shoot went better than the previous ones. The daylight shots turned out pretty good.

http://www.flickr.com/photos/wakesho...7624699293133/
http://www.flickr.com/photos/wakesho...7624699293133/
http://www.flickr.com/photos/wakesho...7624699293133/
http://www.flickr.com/photos/wakesho...7624699293133/

But the eve shots with the flash housing were not what I hoped for. I am used to shooting with a long lens, and setting the shutter at 1/1250 or 1/1600. When shooting with a Nikon flash, in mixed light, at shutter speeds over 1/320, the high speed synch really weakens the flash.

On these shots later in the eve...I was at 1/1600...and the flash was way too weak...

http://www.flickr.com/photos/wakesho...7624699293133/
http://www.flickr.com/photos/wakesho...7624699293133/
http://www.flickr.com/photos/wakesho...7624699293133/
http://www.flickr.com/photos/wakesho...7624699293133/

For these sunset shots, I dont think the difference between ambient and the subject is enough to drop the shutter to below 1/320 and rely on the flash to stop the action. So the question becomes how low can I go and still stop the action?

I tube shoot with a 17-35mm Nikon f/2.8 about 6'-8 from the rider (when on the water) Up close to the rider like this...do you think I can stop the action at 1/800? How about 1/600? There is no way to tell while shooting with a water housing how the shots are turning out.....so I am looking for any advice here that will save a wasted shoot...

What I want to do is catch a eve light like this...
http://www.flickr.com/photos/wakeshots/5019648816/
...during a tube shoot....and get flash exposure of the rider for an effect like this...
http://www.flickr.com/photos/wakesho...7624234683057/

Oh...and thanks to Joel, Ryan and Jess for being photo subjects!