Sure! You can buy electric water pumps fairly cheap online. They normally come as 12V devices so you'll have to use a relay and a 12V supply, and some sort of circuit to drive the relay using the 3.3V GPIO from the Pi. You can control the pins using Python and the RPI.GPIO library. I personally use a Zerodis DC 12V Homebrew Beer Pump (although there are about 50 other types on Amazon) and a length of hose pipe which goes to a rain water bucket that I've put beneath my drain pipe. The other end goes to a sprinkler. Super simple system that took only a day to set up, so I highly recommend something similar for all you absent gardeners.
If you're looking for some serious power, that system I described will still work. The Raspberry Pi is just there to control a relay to complete whatever circuit you're powering. You could get something like a Draper 56225, which manages ten's of litres of water a minute; something I've been considering getting for a hydroponic setup I'm looking to build. Then you can pick up something like an Energenie Raspberry Pi-mote, which is basically a mains power controller you can address with your Pi. Plug your pump into that, and then power the pump with the Pi whenever you want to water your plants!
