Electrocution Risk in Swimming Pool
There are plenty of mains ac powered submersible pumps (using proper rubberized cable) and, if I thought for one second it would kill my prize koi carp (many of them I might add), I would be very worried. Added to this is the need to add RCDs (or GFCIs in the US) - the instant there is a hint of an insulation  breakdown the RCD (GFCI) will trip the power.

I might add that I have allowed my son to swim in my pool and did not worry.

So no, I don't see an electrocution risk from an ac powered submersible pump and batteries are going to be no worse.