Ice not melting in water

This might just be because there's so much there. It takes a lot longer to change the temp of a 10 ton stack vs. the ~500 kg a dupe can carry. You might try spreading it by building a couple storage bins where you limit how much they can hold.

it just takes time.  Be patient.

Could you upload your save file? Chances are it's just melting very slowly - you're dealing with a lot of mass there. Looks like you've got ~30,000kg of ice melting in only ~1500kg of water.