Excess power into a water heater is a "maybe one day" thing for me. No experience with it. You need some kind of alternate means of control if you're using it for "excess", i.e., you only want it to come on when your battery is in float and your panels can produce enough to store it.
A simple thermostat would allow it to come on whenever the thermostat says so.
Yep, that is actually the guy where I saw that he was using straight PV to power a DC electric water element.check out davidpoz on water heating