3.3v sensor to switch water pump
Use a logic level (Vgs-threshold &lt;= 2V would be good) MOSFET (N Channel for a low side power switch, it's the easiest). 

The following diagram is something I put together to teach my robotics club how to do basic power circuitry, in this case to drive a 12V DC PC case fan. In your case, the fan motor is just a 5V motor, but still is essentially a fan/motor. The protection diode shown is still important. 



In this case the "switch" is your 3.3V sensor output. The PIR sensor output may not be high for very long, you should perhaps use a 555 timer and some buffers/inverters to keep the signal to the MOSFET high for longer, with the RC constant as something reasonable for your application.

Depending how your 5V rail is supplied to the pump, you may want to put some bulk capacitance such as a few hundred microfarads, or even a few thousand, to prevent it from drawing the line low during current transients as the pump turns on. This same advice holds true for using servos, and many other motors. Always buffer the power supply with local bulk capacitance!