First of all, be careful with the temp adjustment. Even though it may feel good, setting it too low could cause hypothermia. Similar to how you have to heat a waterbed.
Second: Using a voltage regulator for a pump is quite inefficient. When set to low, the regulator will generate quite a bit of heat. Also, note that the pump may not start up if the voltage is too low - this will take some experimenting to figure out. But if that's still something you want to do, a simple LM317T adjustable regulator with a pass transistor can be cheaply made. The sample circuit can be found in the
LM317T datasheet.
A better solution is a PWM fan controller. Many computer fan speed control modules use this method. Just have to find one that is rated for ~1A. Heck, you could even rig an ESC to do it, but would be more expensive and convoluted.