sure, thats one way, provided that the dimmer uses resistance instead of a zero crossing detector (i.e. a cheap one will work, an expensive one that can run floros wont) you will be turning some of your output into heat, so it will drain your battery more than the right sized pump, but no more than the pump itself does without the resistor.
you may want to look into home-built PWM controllers for computer case fans. they may cause your pump to chatter so you would have to experiment, but they have the advantage of actually blocking the flow of current rather than giving it off as heat.
allan