Hi. Great work!
I owned my own marine canvas shop and can recognize excellent craftsmanship. Well done!
This a great idea. I was looking for something like this, but didn't think of it. A few years ago in ME I was hiking through some singletrack and the wet trees, etc were drenching my shorts.
At the time I wondered if they made some type of silnylon shorts for that purpose. Never thought of a skirt.
One thing I can suggest, if you don't use it already, is to use sticky-back, or "back-to-back" tape when preparing the hems. This can help keep the slippery silnylon in check somewhat. You can buy different adhesive strengths and widths.
To keep the snap working well, you can run just a little petroleum jelly on the inside of the top snap, where you can see the little ring inside there. This method is used on boat cover snaps, particularly on salt water boats, to help prevent corrosion.
If, over time, the 6 layer silnylon reinforcement starts to come lose you could always use a stronger material instead. Looks like it stand up, though.
Hope this helps. Congrats on the great job!