And so ultimately, despite my details being wrong about currents, the notion of stringing miles of panels together is probably not a good basis for rejecting the panels over the canals.
In reality, you shouldn't string a lot of panels together for other reasons. An obstructed panel doesn't just not produce energy. It acts as a resistor, _wasting_ energy that flows through it.
You would have short strings of panels and small string inverters built every ~100 meters along the channel. Inverters will also connect to the high-voltage bus cable, likely buried in a trench along the channel. And most channels in California are not gravity-fed, so they already need pump stations along the way and have power distribution systems for them.
While having 12kV going through panels is not safe for many reasons, having 300V at 10-20A is completely normal.
12kV is impractical because these voltages can jump quite far, and you need a lot of insulation for them to be safe.