Originally Posted by Cider123
One thing you could check out is something called a "swamp cooler". It is basically a big tub that you put your fermenter in and then fill with water. It is a cheaper way to moderate temps for your beer rather than buying something expensive. It increases that same concept about large quantities of water resisting temp change. The former science teacher (and dork) in me would refer to it as water's high specific heat.
As I understand it, a swamp cooler works by evaporation; a towel or tshirt on the fermenter sits in the water and wicks it up. The evaporation draws heat from the beer and cools it. All that said, you're right that just putting your fermenter in a tub of water will protect it from any temperature swings.