You could take DonR's thoughts to the bank. You would have to modify the cooler, like cut a hole in the side and mount the Peltier in the side or floor of the cooler and silicone around it to seal it, then insulate or even better, heatsink the other side. Check out the rather large one here:
Gigantic 62mm - 545 Watt Thermoelectric Peltier Cooler - eBay (item 310104062553 end time Jun-24-09 15:45:03 PDT)
That would be a good solution, however, you will want to have something that moves the water around inside the cooler, like a propeller. Otherwise you may freeze the water nearest to the Peltier device. Good luck.