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I can't imagine a leaking battery sinking to the bottom of a waterway is a good thing. Probably some kind of chemical hazard.

The battery compartment is typically waterproofed, and freshwater is considerably less conductive than saltwater. You might feel a tingle but it wouldn't kill you unless a really severe collision split the battery case open, in which case the collision probably already did you in.

Good answer, you know the subject well, wish I had stronger voting power for ya. Someday soon I will!