I also use the "water in tub" method in my unheated basement. Using ice bottles, it's easy to regulate temps in the summer. In the winter, I use a adjustable 100W aquarium water heater. The water heater is very useful to bump up the temp for belgians and for a lager's diacetyl rest.
I also add a touch of star san to the water if it is going to be in use for more than a week.