Snotter, I think you have it backwards. Fresh water is non-conductive (not like metal) and shouldn't give a good return. Saltwater is more conductive (like metal) and should give a better radar return.
Has nothing to do with electrical conductivity. It is all about the ability to reflect the microvave signal and bounce it back. Not all ice has the same density or radar wave reflectivity. Vertical surface to the radar reflect the signal better and paint a better image on the display. Remember our radars will also reflect off water and wave faces, in fresh and salt water, and we often struggle to "tune out" or minimize the sea clutter.
As Tim said the low growlers are hard to see on radar. This is often because they have been washed by the ocean and are smooth and low usually with no vertical angles to reflect off. If the salt has eroded them then they can become more absorbing and less reflective but will often still reflect if the proper angles are there.
Neutrally buoyant bergs are impossible to see on radar and present the most danger because they float just at or near the surface but rarely poke above it. Icebergs on the other hand can often be seen for many miles, especially when new or recently calved. As time goes by though they too get smoother from ocean wash and may not reflect as well as they lose many of their sharp edges and angles that help reflect the signal..
One trick I learned is that you can often detect a non-reflecting chunk of ice by the lack of sea clutter in the radar shadow behind it so sometimes even poor reflecting large objects still give us usable screen data. I have never tried this with large ice. I have however once picked up a huge floating mass of weed and garbage due to the lack of sea clutter. This is one reason I tend to leave some clutter on the screen and not completely "sterilize" it.
While there are materials that make better or worse reflectors for radar, ice can certainly be a decent reflector. Wood and fiberglass can be poor but even these can still reflect..
The shape of the ice has a lot to do with this as you'll need a surface that is in the right angle to bounce back a signal. I have picked up ice on my old 4KW Raymarine non-HD with no problem but this was new ice not old stuff that may have more porosity, be more sea washed and thus may not reflect well. Not all ice chunks could be picked up however because some were just poorer targets, due to angle, like many boats are. Ice in general is hit or miss due to the shape or reflecting angles not necessarily it's ability to reflect a signal.
The danger of ice, like boats, is that you will not pick them all up. This is why boats use radar reflectors, because many are poor targets. Ice chunks don't use reflectors...
. So sorry