The H2O comes from the reaction.

NaHCO₃ (baking soda) + HCl (hydrochloric acid) → NaCl (sodium chloride) + H₂O (water) + CO₂ (carbon dioxide)

Water is created. Now pass that through your entire digestive tract full of HCl. Now H20 pops up, uh, where it’s not normally in the pipeline. Does that make sense? You now have water where your guts weren’t prepared to process it?

In any case, I shit straight liquid if I have to do this trick.