We used to shoot a pound-sized cylinder of sodium into a sizable river out of a mortar made from a carpet tube (the thing carpet is rolled around, heavy cardboard) charged with black powder.
It's fun but mildly guilt-inducing until rationalized as follows: a pound of sodium is about 20 moles. Dissolved in a cubic meter of water, it makes a 0.02 molar solution, which isn't very strong. There's far more water than that involved, and the water is somewhat buffered with whatever's dissolved in it, so the change in pH, even fairly close to the reaction, would be very small. Of course, the reaction is rapid and violent, thus not good for anything that was close by, but a river is a big place.
Edited to add: I think this would be a very bad idea in water without a current, because concentrated parts of the NaOH plume would be dense enough to sink to the bottom, making it very unpleasant there until it dissipated.