Brian's post is interesting and would be what you're looking for. Please note it's only 500 W at 120V. Most 120 V heaters are 1500 W.
All the 300 W and 500 W ratings mean is that it'll take longer to heat your water.
From your use descriptions, it sounds like what you're doing is elective, and not necessary.
When you've finished motoring to an anchorage, your water will be hot. After a day at anchor, not more hot water, but your wind machine will provide your hot water heating. No wind? Use your inverter, but that's not a good idea unless you have a HUGE house bank.
I wonder how the combined 12V/120V heater element is wired to have two sources to the same element without MANUAL switching between the two different voltage inputs.
If it was my boat, I'd keep it simple. Wind dump to house bank. Either choose 12V or combined 120V/12V heater element. Do the switching manually with a simple dump. Forget using the inverter for hot water.
Every time something else is added, it requires a backup, so plan on bypasses around what you add or backup "pieces" of "stuff."
Toggle switches are good bypasses!