I know that water is most dense at 4C. It then starts to expand as it cools further and turns to ice. I would expect that very cold ice is more dense than ice at 0C. It seems possible that we could set up a system of warm water and very cold ice, such that the ice would sink in the water.
But then again, will the ice melt and form a layer of less dense water around it? Will the water freeze to the cold ice making it overall more bouyant?
So here is the overall question: Can ice sink in water?
The short answer is yes, absolutely. I am searching the SDMB archive for a thread which has a weird video of ice coming up out of the sea and over an embankment. The ice was under the water noton the surface and it was being puched up by the incoming tide. It’s a really cool video but I cannot seem to find it. Someone posted it about a year ago…
Not quite what you’re looking for, but. . .
There is one type of ice that is denser than water (link).
Ices II,III,IV,V,VI,VII,VIII,IX and X are also denser than water.