My well water is never warmer than 65 F, so I make frequent use of the microwave to adjust temperature. I have on occasion put a 500 ml container of solution in the freezer if it was too warm. I think ice in a ziplock bag is the best suggestion so far, unless someone knows how to run a microave backward.

There are in fact solid state cooling devices related to bimetalic strips that might be made submersible. These are used as coolers for electronic equipment.