Been following this thread for a bit and there doesn't seem be a water heat exchanger heater avail yet outside of Alibaba and shipping to the states adds another $130-ish the price making it a $360+ range purchase.
However, here are a few links for you guys already running these heaters and need the exhaust heat exchanger the one guy posted on his site with the UK auto parts. Pool exchanger work the same way so here you go for your exhaust heat exchanger needs. Pick a size cuz I do not know what size the exhaust pipe is but the water connections are standard pipe threading from what I can tell.
Exhaust Heat Exchanger
Finally, this is what I am planning to run on my Landy for hot water with a separate tank for showering/washing only. Plumb this into the coolant system and you have heat exchanger capability. I for one run my truck for a few minutes daily anyway for sanity check when I am off the grid, you can circulate water with a pump from Home Depot/Lowes, etc... and heat as much water as you want from coolant temp specs on your rig. 5 minutes of run time with circulated water and you can heat your water tank to shower (almost boiling hot) temps. Put your pump on a thermostat switch and it will shut off when it reaches the right temp of your toasty buns choice. Any 12v pump of flow rate choice should work just depends on what circulation rate you want. You can also buy drop in submersible pumps about the same price if you want to run a 5 gal bucket or reservoir/river feed of sorts.
Vehicle mount by my parts list is about $250 one time installation and I have on demand hot water anytime I need off the vehicle. Advantage is you can run this off a single battery with the vehicle running during hot water circulation for the 5 minutes or so that you need to get the engine to operational temps so there is no battery drain issues at any time. One of the very few items I will run on my start battery only since the vehicle will be running while I am circulating water to get it to my desired temperature.
Coolant Style Heat Exchanger
12v Lippert Frest Water Pump