Nose clips which are worn while swimming have some anecdotal success. We called the Indiana Natatorium, site of the Olympic Trials last year, and the model for the pool in Sydney to answer this question. When the surface is level and calm, the swimmers get less resistance and aren't hit by other swimmers' wakes or waves. Everything is designed not to let the water splash back into the pool. The Natatorium is 9.5-10 feet deep throughout.
Turbulence goes down, and that depth takes surface roiling right down.
Special lane markers, commercially available, also minimize waves and wakes crossing lanes. Side troughs keep water from sloshing back and forth on the surface of the pool. Lastly, they also minimize surface turbulence by placing all the recirc jets near the bottom. But there's no real way to measure or prove that. Check out the current issue Search the best of the web.