That's funny! 128 gallons!
I guess my point wasn't clear. The reference to 128gal was that in order to boil your 6gal tank dry, you would have to boil all of the water on the boat, because as the heater tank boiled off, it would be replenished with new cold water from your water tank.
The reason you aren't hearing about 12Vdc heater elements is that there are only 6 or so people on this thread replying, and most of them aren't cruising or do not have excess solar. Doesn't require a fall from a turnip truck. I know many people using them - they are very common on boats with windgens as load dumps. People with solar use them less so because most don't have excess solar, and solar can be regulated by just shutting off the controller output - unlike wind.
As I mentioned, they aren't terribly efficient, particularly for excess solar which is usually late in the day. However, they are viable, and people are using them. Many don't have AC power on board, so replace their tank elements with DC ones and take whatever excess DC comes their way to supplement their engine heat.
I read your post as asking about the possibility of putting some excess solar energy to use heating water and tried to focus answers on that. But it seems like the consensus is that you should have a solar shower, or a solar heat panel, or a heat sink, or a new battery bank. And that all of the cruisers I know using DC heating elements are gullible newbies being taken for a ride by the big corporations. It would seem that none of these responders actually used physics to come to their conclusions.
In a final attempt to actually be helpful, here is how to determine how much energy it takes to heat a gallon of water:
Q=mc∆T, where m=mass of water, c=specific heat of water, ∆T=change in temperature in Celsius. The resulting units will be in Joules.
Since the mass and specific heat is constant for water at sea level, this simplifies to Q(J)=(15,818)∆T. Converting to Wh for solar:
Put another way: ∆T=(Wh of excess solar)÷4.39
Now, if you know how much extra solar power you have, you can easily determine how hot it will make your water.
For instance, if you have 300W of solar, and 3hrs/day to dedicate it to heating water, and the water is at 27C (I've moved you down to Mexico), then those 900Whr of solar will heat your 6gal of water to 135F. Of course, that is at nominal panel specs for the solar, so this should be adjusted to ~40% to accommodate late day sun and imperfect conversion. Given this, you can expect those 900Whr of solar to heat 6 gal of 80F water to 105F.
So the question is how much solar do you have, and how much excess do you expect?
The question is not which turnip truck you fell off.