Chemical/medical question

Sodium or potassium would explode. Phosphor sends things up in flames. The only problem is, is that these elements react rather well in atmospheric conditions so they'd have to be insulated.

If you need something immediately lethal, the only thing i can think of would be some actual high explosive, triggered by a primer with something like magnesium phosphide or some alkaline metal. Quite a lot of high explosive at that to make sure it's lethal. I can't think of any substance that ignites on contact with water and is powerful enough alone to cause a lethal explosion.

The other option would be something highly toxic. Hydrogen cyanide for example will result from a reaction of an appropriate cyanide salt and an acid, so using a mixture of a crystalline acid and cyanide salt will produce hydrogen cyanide on contact with water. I suspect that finding this at a crime-scene would result in a full scale terrorism investigation though.

Acids are not all that useful for this. They can easily cause dangerous chemical burns, but don't just dissolve a whole human or are even quickly lethal. Not to mention that a whole bathtub full of water will dilute any acid to harmless levels. (And even if it didn't, you'd expect people to notice when they put the first toe in the tub, not only after being completely submersed in the acid)

The cyanide was my first thought. The gas chambers used for executions involved a solid substance that dropped into a pan of water under the chair to which the executionee was restrained. The water activated the HCN gas. Zyklon B, used in the Nazi gas chambers, worked similarly but was activated by water vapor in the air.

Thallium

http://en.wikipedia.org/wiki/Thallium_poisoning

Effective ingested or through skin contact, if there was enough in the water it would likely kill the subject before they could get to a hospital and receive the proper treatment (Prussian Blue).

It has a long history in fiction and reality as an effective poison for murder and it's a powder (in it's most toxic form) so could easily be incorporated into a bath-bomb.

Yep, Thallium would certainly work to send someone to the hospital, but i don't think it'd actually be fast enough to kill. Afair even severe Thallium poisoning takes several weeks to be lethal, and toxicity is quite low. You'd probably need a bath bomb made out of almost nothing but thallium sulfate (or other thallium salt) to get enough thallium to be absorbed through the skin.
A whole bathtub full of water is a lot to dilute poisons, same problem as with acids. (only you can get poisons with higher toxicity, but not better acids)