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!