Zodi is a good idea but for me at least the weak point is the pump. I am on my third unit and have given up on Zodi. My opinion way too expensive for what you get.
I have used passive heating units from Nemo (built in pump, not very efficient at heating water tho) and several of the other passive heating brands which quite frankly I threw away after a use or two. The Nemo unit is well built and has a built in foot pump for the shower head. This is pretty convenient.
The best unit I have found for all around cost and use is the Stearns 4 gallon solar heater unit. Forgot how old mine are. The originals had a pretty much worthless plug to fill water. Some years ago they came out with a new model with a screw on water plug and much improved shower head on / off head. If you can get a few hours sun you have decent hot water on a river like Grand Canyon water will be too hot especially in the summer months. The unit I have has a lot of trips on it and scratched up but still works like a charm. The plastic is pretty much bomber and has zero leaks after years of hard use.
On river trips I use the stick type water cannons to fill showers. I have a head hi tripod we set up for showers. Fill them all in the morning and attach to rafts whose drivers are willing to have them aboard. We normally have a couple folks assigned to shower duty. They put up the tripod. If you want a shower, get a full unit from a raft and take it to the shower tripod. In the morning the shower crew takes down the tripod, brings empties back to be filled and we are good to go for the next cycle.
If Zodi would come out with a decent pump that worked, my opinion that would be a good way to go but expensive and the little canisters of propane are a pain to deal with. In the meantime the Stearns units are pretty much free hot water. Since most people only take river showers when the sun is out, things work out.
Your experience may differ.