I believe they only operate with high impact force (the manufacturer's notes don't mention water activation... only "crash situation"). I don't have any values for the g forces required, but I'd imagine they would need a respectable jolt to operate them (certainly higher than bumps sustained during aircraft ground handling). The sensor may be oriented in a particular plane/axis, so a mid-air breakup may not trigger it.
The ELT used in our aircraft must sense a 5g force in any direction (spherical sensitivity) for a sustained duration of 11 milliseconds.