While the water is being collected have the other students agree on an interval to collect the temperatures. Have the groups draw charts for the data collected. Determine which will be the dependent variable and the independent variable, time or temperature. Now students can draw the axes to graph the data on. After about 10 temperatures have been taken have the groups plot their data, draw a line of best fit and compute an equation for their line.
Share results with the rest of the class.
A field trip to CPL before would be a good way to set up this lab. Obviously the temperature will not drop indefinitely so ask students why this is. You can also discuss different variables that would effect this experiment and give different results.