If you look at the amp ratings of the Indel hot water heaters they're 750 watts, whereas the US makers (e.g. Raritan) are all rated 1500w.

This makes sense if they're using a heater core that's really designed for 220v European (as you wrote above). My boat came with an Indel unit and I found I can run my hot water heater at the same time is my 40A charger using my Yamaha EF-2000is generator (rated 1600w continuous). I don't mean to kick off a generator thread but after two or three cloudy days on the hook there's sometimes a benefit to running it for a few hours (in the day when it's less likely to bother others nearby).