Flash Heater Improvements
These Triton Heaters get rave Reviews. I've used an Electric Flash Heater I installed in a House, and have used plenty of such Heaters in Worldwide Travels. Big Wire and 40 Amp Circuit Breakers @ 230 VAC [208 VAC actually leg-to-leg]. Flash Heaters are how the World heats water. The 240 VAC types at Scottish and Campgrounds fed by Coins in the Shower with you kinda weird me out, but they're on GFIs obviously.
A handy way to cut flow is put a Nylon Disc in either end of the Shower Hose. Drill a small hole in it. Check flow. Increase the hole size until you like the flow/water conservation tradeoff. This is how Commercial Shower Heads are made to be 'low flow'.
Another trick is use a nice, hard-mounted [immovable], Ball Valve w/Handle. You can mark Handle positions relating to flow - 2 o'clock, 3 o'clock - and so on. Then, you can get back to those settings repeatably.
One Reviewer stuck the Pump intake in a 55 Gallon Drum of water and the whole Hunter's Camp could Shower up nonstop. Intake from a Stream should work, too. Just don't drink the Water; this is how much of the World showers. That is, potable Water is kept separate from Drinking Water.
Any Flash Heater I've seen has an internal flow Sensor that has to see some minimum flow before it turns on. So, too small a flow won't trip that Sensor to start the Heat Cycle. Just something to be aware of when playing with flow-reducing solutions...