Firework's fuses are made to continue burning under water. I have heard, possibly apocryphal, stories about cherry bombs being flushed down toilets and exploding pipes. There was a scene like that in either Problem Child or Problem Child II.
If you flush a cherry bomb it makes a big mess, sometimes breaks the toilet, maybe more. Saw it in junior high school before kids were searched going into school.I'd be careful on ice, though.