The circular plastic connector that plugs into the traction battery is fitted with heat-shrinkable tubing lined with adhesive which makes a pretty good water-tight seal between the connector body and the plastic loom that protects the wire harness. Unfortunately, the upstream end on the wire harness often leaks, and the seal only serves to hold the water in. The connector includes a rubber weather-pack around every single wire, but because the harness slopes up, the water in the loom is pressurized by gravity and eventually works past the seal and causes galvanic corrosion. I have seen a few cases in which pins actually broke off inside of sockets and could only be removed using a specialized tool: a paperclip.
I don’t think that the water that gets into the connector goes on to enter the battery– I don’t think any pressure would build up on the weather-pack on the bulkhead-mounted socket, the battery-side of the connector. But water definitely finds a way in through other flaws in the battery case seal.
Every Think that I have ever done any battery work on has that heat-shrink broken loose from the connector to allow water to drain.