Send Clowns, was this expert talking about aviation matters? My "personal" expert (
) informs me that, whilst it is useful to maintain a steady throughput of liquid to keep the kidneys functioning, much of the effect of diuretics is lost since the liquid intake is lost too rapidly to replace that lost in both the bowels and by sweating. I am also assured that you can sweat a significant amount without noticing it - i.e., with no noticeable damp patches under the arm, moist brow etc.
Most diuretics (dependent on type, concentration etc.) result in a net loss of fluid in the body.
I am therefore very sceptical of the claims you heard and would suggest that water and fruit juices would also be needed to maintain best hydration.
Do we have an Aviation Medicine specialist who can assist here?