When I was on the Colorado in April we had several seriously windy days, and on one particularly windy layover day I just wanted to get out of the sand and wind. I put my Kelty tarp over my bimini and attached it all over with straps, including straps diagonally inside to stabilize the bimini top because I was worried that the gusts would break the pivot attachment points. It held up to some super hard gusts, maybe 50-60mph. It was so nice to be out of the wind that I decided I really want to make side walls also (I sleep on my cat), so I've already put a lot of thought into it. I've decided to use panels with Velcro but I haven't sewn them yet. I don't ever plan on rowing with side panels up so I wouldn't have openings for oars. I want the panels long enough that I can get to anything on my boat without moving or adjusting anything so I'll have them extend to the cat tips.
For a shower I've gone to using a small well pump and a motorcycle battery. My first shower setup was just a little battery operated Zodi camping shower, which was fine, but I didn't like it because I was limited in how far away I could place the battery pack from the pump, the batteries weren't rechargeable, and the head pressure on the pump was not very good and limited how high up you could effectively place the tiny shower head especially as the batteries started to run out of juice.
For my new shower I found most of the parts on Amazon, here's the well pump: Amazon.com : Rule iL200P Marine 200 GPH Inline Submersible Pump (12-Volt) : Boating Bilge Pumps : Sports & Outdoors
I made all of my connections quick-connect like this:
Amazon.com: Allstar Performance ALL76232 Universal Two Wire Connector with 12" Loop: Automotive
I made a shower pole out of pvc pipe that I connect to an NRS knuckle on my frame, and fashioned a shower head holder out of some misc parts. You could throw a tarp over the bimini for privacy but I've found that for most river trips privacy just isn't something anyone cares about that much. I had designed an entire enclosure out of pvc pipe for my Colorado trip last year but 1) it was a pain to set up and 2) almost nobody used it, so I've abandoned that idea, especially now that I have the bimini top this year. You could always throw a tarp over the bimini for privacy, or use your towel to block part of the view. Once I make my side panels I could use that as well.
I mounted a 20 watt solar panel to one of my dryboxes, mounted the charge controller inside the box, and ran the wiring from the solar panel into the drybox where I store 2 sealed motorcycle batteries. I can run the river with a battery charging, and another charging up phones or camera batteries or whatever.
I'd be interested in what you come up with for your side panels. Your post reminds me that I need to get to work on mine!