That's fine, if you have a course on the river. We only have a course at our private site on a lake, but we do a lot skiing on the river as well. Trick skiing is actually the bigger concern for use while using ZO on the river.
As expected, the river was only flowing about 0.4mph, according to ZO last weekend, so I didn't really see a difference up/down stream. If I was able to ski I could probably have felt that difference on the trick ski.
However, the real problem was that we didn't have the manual for ZO and I couldn't figure out how to get in recreation mode anyway. So there was lots and lots of beeping during the slalom runs.
On a postive note, both women we had skiing said it was the nicest wake they had ever skiied. One was 15off, and the other 22 off...both at 30mph. Both commented how the boat had no give when they turned...they are used to PP.