Perhaps a piezo element might be a place to start - waterproofing it might be tricky, but they respond well to impact especially if mounted on
slightly flexible material like a plastic sheet - perhaps epoxied on with epoxy covering the piezo disc and the connections to water proof it.
They are cheap and sensitivity is set by choosing the right resistor in parallel. Don't be confused by piezo beepers, you want a raw
piezo element like this: http://www.maplin.co.uk/piezo-transducers-3202
They are quite brittle, so make sure they are mounted to a stiff enough sheet. In the past I've used these loosely stuck onto the
side of a drum to pick up impacts - works well.