The overwing exit slides are not designed to float or be detached and normally would only be used for land evacuation.
In fact in a ditching the overwing exit slides will probably
not even inflate, as the aspirators for the slides are located underneath the wing:
Off-wing escape slides are permanently armed and deploy and inflate automatically when one or both of the window exits on that side of the aircraft are opened, from the outside or the inside. The deployment and inflation sequence is achieved in approximately 5 seconds.
The aspirators that draw in ambient air to inflate the off-wing escape slide are located just below the wing. In a ditching, the off-wing escape slide should not inflate as these aspirators are likely to be under water.
In the unlikely event the off-wing escape slide does inflate, it can not be used as a raft or flotation device and can not be disconnected from the aircraft.
Interesting that in the US Airways Hudson River incident the overwing slides did achieve at least partial inflation. Does anyone know the exact location of the A320 overwing exit escape slide aspirators?