This has been asked quite a few times before.
As its only in the wet, it means its a water related problem in relation to the belt. Water + Rubber = Slippy.

but not in a weird way!
Thus we must conclude that the water is making the belt slip for the power steering pump. This has happened on a few cars I have driven of various different marques. Remedies include driving slower in the rain and fitting a splash guard if you are really that bothered by it.
If you had a slack belt, the problem would occur even in the dry.
Hope that helps!