Can the overwing exit slide be used when ditching in an A320?
The over overwing exits are secondary exits in a ditching and they should be used only if it becomes absolutely necessary to use them. The reason is because, the escape slides of the overwing exits are just slides and not rafts as they cannot be used as long term flotation devices. Moreover, unlike the slide rafts, the escape slides on the overwing exits cannot be detached from the aircraft. If the aircraft sinks, it takes the slides with it. The rafts can be separated away from the aircraft and can be used as small dinghy to hold the passengers. Furthermore, the rafts also have a loop to which you can attach the survival kit.

Here you can see that the A320 has four slide rafts (fwd and aft doors) and two overwing escape slides.
If the usage of overwing exits become necessary then they can be used in a ditching. In the Hudson landing, they used the overwing exits because the aft exits were rendered useless due to the considerable amount of impact damage received by the aft fuselage of the aircraft. Nonetheless, there are ditching related safety devices in the overwing exit. This is because in a ditching, the wing may get slippery. So, there are life lines that can be attached to assist passengers to escape the aircraft over the wing. This is what that yellow hook you see on the A320 wing is used for. So, yes wing exits can be used in a ditching, but they remain as secondary exits as long as all the other four exits are in working order.