Unsurprisingly, when you’ve got skis on your feet, glide ratio deteriorates.
In the example below, you lose about 0.6 pt of glide on a B wing. This depends on your skis, but also on your flying position. We can see that the most favorable position is to put the skis as vertical as possible. The reason is that, despite the increase in ski drag, most of the pilot is in the wake.

Another good (but uncomfortable) option is to set the skis at 90° with low incidence (a bit like a wing). The loss is then more than 0.3 pt.