Epsilon r of Salt Water
I would say, the real part of epsilon remains the same, but an imaginary part with the parameter sigma*frequency*mju0 gets there also, making the refractive index a complex number, thus represanting the attenuation of the electromagnetic field.
I guess, finding an instrument that measures conductivity wont be hard, or you might even calibrate your own setup with copper rods. check out **broken link removed**
where they state
Absolute pure water - 0.055 µS/cm
Distilled water - 0.5 µS/cm
Mountain water - 1.0 µS/cm
Most drinking water sources - 500 to 800 µS/cm
Sea water - 56 mS/cm
Max for potable water - 1055 µS/cm