Why copper wires submerged in salt (or fresh) water oxidize much faster while they conduct electrical current? On a lot of flooded vehicles (salt/fresh water), I've noticed that wire that carries more current is always the one with most destruction (at the connector/splice), in comparison with lower current carrying wire. Wire turns black on the outer layer, accumulating green/white oxidation(?) on top of it.

How does electricity affect the rate of oxidation?
Electricity in a wire submerged in water effectively turns it into an electrochemical cell, and then it is no surprise that the anode gets oxidized pretty quickly. A modest potential of a few volts would suffice to oxidize any metal, even gold. This works for AC as well (a wire oxidizes during the positive half-wave and then does nothing during the other half-wave).