You can adjust the speed of a motor, within certain limits, by controlling the voltage applied to the windings. Since the components of most motors are designed only for a specific amount of electricity that means you can slow a motor down but you can't make it go faster since applying more juice would overload it (not safe).
Most dimmer switches are designed to dim a lightbulb or chandelier fixture, which is maybe a 100-150 watt fixture (max). A pool pump needs a lot more juice; my pump has a maximum draw of 13.8 amps at 115V, or about 1,600 watts. I am sure there are rheostats available that can handle bigger loads, but it is not something you are likely to find at your local