The electricity wouldn't bother you, but;
that's a heavy object - if it should hit you,
and the acid in the battery is quite strong - if it splashed on you.
Unless it hit you, nothing.
ANSWER:No. nine volts isn't enough to harm you. The water would disperse most of the little amount of voltage anyways.
NO unless it was used in a salt water pool even then it is unlikely that it would do any harm in most situations.
I've never tried that, but I am pretty sure the battery would no longer work.
no
once
Nothing. you'd get wet, and the battery would get wet, but noting else unless it was plugged in to something, then you may get shocked.
No
No
It will rise of course. The metal that is thrown overboard has mass, and displacement.
No
No, if the water chemistry is right, there will be no impact on the pool water chemistry.