Some waist harnesses are bigger and offer more back support and comfort and they also scoop up more water when ya on ya back. If you look at alot of the top unhooked riders ...they use smaller waist harnesses that are not so tall in the back. These harnesses must provide maximum mobility for doing all the unhooked moves.They are probably less likely to act like a sea anchor when you are on your back because there is less harness surface area scooping up the water.
www.mysticboarding.com/ Go and look up the Mystic Waist Harness - DRAGONSHIELD specifically for the unhooked rider
It would be interesting to see what everyone else has to suggest?