I don’t believe that device may be water proof, just resistant on the surface. If it’s submerged it may likely stop working.
A better way would be to use a water leak sensor with a wire and prongs like the Everspring/Utilitech water sensor which a long wire that you can attach the side of the tub to inform you when it’s full. Do note that some batches of this device are problematic, but otherwise an excellent sensor.
Alternatively you can fashion your own custom sensor using a Monoprice Door/Window sensor, it has an external wire switch to which you can attach wires and use it a water sensor, however you’ll need a custom device handler if you’re going down that route.