Actually a small exhaust-to-water heat exhanger would provide quite a bit of hot water.
I considered it but I am concerned about the steam flash problem and the other complexities involved.
My system is up and running fine in my garage. We can't hear it running in the house nor can our neighbors.
Here's how I connected the 220 VAC generator cable to the W/H.
220 to 220.
Open one wire of the W/H to the center of a SPDT swtich. One throw to A/C from panel, the other to a SPST switch.
The other end of the SPST switch to Neutral.
(The prongs of the generator plug are of course energized and an plastic insulator cover was fabricated for safety - no touch.)
With grid power, everything normal.
No power, flip off Main Breaker, throw the SPDT switch away from panel to lessen the load on my 3,000 watt generator.
I also flip off all other 220 breakers. Generator is used only for 120 devices and the small breakers.
Plug in cable, start generator. Generator feeds house through W/H breaker.
Hot water needed? Throw the SPST switch to neutral and the H/W heater is using 1/4 the power (~850 Watts) on 120 VAC than it uses at 220.
Simple, efficient, and for me, quite safe.
For photos, email me.
YMMV