Yeah, I've considered that and things like that... The closest thing that I came up with was a camper or portable instant water heater like the Eccotemp L5 or L10.
The thing about these units - I wouldn't exactly call it a "problem" - is that they run on propane. That means that I'd always have to carry around and maintain a propane tank or two (since I'd surely run out before taking it to get it filled). Sure, they'd give instant hot water, but I preferred the idea of an engine heat exchanger due to the invisibility of the system (it's all buried in the truck) and the fact that there was nothing to maintain, set up, or vent. These portable units have to be set up so they can vent hot gasses properly - which means I'd have yet another piece of gear to set up, break down, store, and maintain. The heat exchanger does away with all of that. It's simply a part of the truck.
Yes, the flip side is that I'd have to have a warm engine on the truck. My dive times between shutting the truck off and cranking it back up again can be anywhere from an hour to a full day... Maybe 2-4 hours is average. But warm-back-up times are always pretty short... Anywhere from 2-4 minutes (only been off an hour or two) to maybe 10 minutes (overnight). It is, after all, a big, honkin' diesel.
I suppose that a hybrid system could always be figured out... And a side-by-side comparison of both systems would be great... But in having to make a decision for the first time, I figured that between cost, complexity, and available resources of heat, the heat exchanger system made the most sense... Especially once I set up a tank in the truck.
One of the reasons why I dive out of a big truck rather than a van like a lot of my competitors do is because I enjoy the bed area as a changing/staging area. I keep the bed relatively clean, and it's constantly getting cleaned by rain and wind (the truck going down the road) - and sometimes even a hose-out. This makes for a wonderful "stage" from which to change in and out of suits, rinse gear, etc... Especially if the dive site is dirty and sandy. Thus, I've always wanted rinsewater IN the bed of the truck with me. This is a great way to make that happen with both hot and cold running water... With very little setup. All I'd have to do would be to connect a garden hose and start the truck - and maybe wait a few minutes for the water to be full temp, as you point out.
Electrical systems like you point out would be relatively inefficient, and need AC110v. I have an inverter already in the truck, but that'd max its power output. My boat has a 12v one, and it works well for the maybe 1.5 gpm it puts out - but is weak and sorta secondary to the system's engine heat exchanger.
For the record, there's a couple of things that I definitely want the system to have...
One is an actual cold side and hot side... You know, a faucet... So the diver can adjust temperature from "too hot" to "too cold" and everything between. A lot of these systems are designed to be 100% of the water supply, and as such have complicated systems for water temperature control. The heater has to heat *just enough* to be the right temperature, and not so much as to make the water too hot. The camper style heaters actually control both the flow rate and the temperature, so really hot water is possible, but only when the water coming out is a trickle. That wouldn't do me any good.
Another is really great water flow. Most of these systems can only operate at like 1.6 gpm. The larger can go to 2.5 gpm - which is effectively the amount of water that comes out of a water saver shower head (annoyingly small). One unit goes up to 3.8 gpm, which is more like the minimum of what I need for rinsing, but it's a propane-powered unit and would require setup, breakdown, maintenance, space in the bed of the truck, and of course, I'd have to carry around a couple of propane tanks for it to work.
Lastly, many of these systems are only capable of raising water temperature 30 or 40 degrees. That means that hose water coming out at 55 degrees would only go to 85 or 95. Normal shower temperature is around 103, and there'd be times where I'd want it 108 or 110 if I was really cold. If the water coming out of the hose was colder, I'd be really screwed. I mean - ANY additional heat would be nice, but if I'm going to build the system, I may as well have it capable of putting out serious heat with serious flow.
The heat exchanger system actually has a warning on it from the manufacturer: Water coming from it could be TOO hot, and essentially the same temperature as the engine - which is normally around 195. Lol. Something tells me that I'd get plenty of hot water there at just about any quantity... 5 gpm at 108 or 105 - pretty much limitless if I have a garden hose on the site - would be an easy demand on this system.
...So yeah - the reason that I want to do it this way is the same reason why I work out of a jacked-up F350 diesel crew cab Super Duty longbed... Lots and lots of whatever it is that I need.
It only makes sense to use the heat that's already there in large quantities.
