Need more details about the water sensor, there are many different options they have. The non-contacting sensor modules can be analog or digital output, and source or sink current. It sounds like you are getting enough voltage to turn on the mosfet, yet the pump motor does not spin, which does not make sense.
The really cheap (contacting) water sensors are one transistor affairs (emitter-follower) so their output voltage can be weak, not enought to fully turn on your mosfet.
I would include a run timer so the pump cannot get stuck on and make a mess, i.e. if the sensor falls off the tank.