Use a motion detector hooked up to a screaming turkey system.
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You can also purchase the crazy turkey sound module 2 for $1
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There is nothing more terrifying then the sound of a turkey
I hacked the turkey module and replaced my car-alarm siren with a super amplified digi-turkey. It's also a great way to pass time watching people jump when you key it from the remote.
Check out x10.com they have several devices that will probably work for you with respect to motion detectors etc. If you check out some beam robotics sites they will have schematics for building infrared tracking systems that may work to aim a water device at a cat.