Wiring on OEM harnesses terminates into pins, which are pushed into the connectors until they click. At the point where this occurs, there can be a very small amount of wire that is not insulated (in rubber) but also does not make it into the pins. Along with that, the pins are exposed metal. There is also a slight gap between the outer diameter of the pins and the actual wire inside of them, depending on the style of pin/connector used, and the gauge of wire used. All of this allows for a small gap, that water could theoretically get to.
However, even without direct water on the wires, with water in the housing/socket, there is high likelyhood that something could get shorted upon contact with the water, and as such, it seems the dealer is on the correct track.