Without warning

[Mesozoic Mister Nigel]

Diluted vinegar is a weak acid, and cheap.

[Fractalbeard]

That sucks.  You should see if you can get hold of Lithium, though it would need to be a fairly large amount to do anything except fizz and fly around the surface of the water.

Lithium Ion Batteries.

Haven't tried it, but charge one of those up, crack the fucker open, toss in water and run, from what my organic chemistry teacher told us in class one day (not quite in those words of course  ).