Some in the past, who have contemplated heating water in this way, have adapted to IR space heating for heating air, instead of heating water. Mainly because of the difficulty of intrigrating a GFCI.

I'm a little confused by what you're trying to do.  I'm not sure what a controller has to do with heating water, unless you're talking about building a thermostat.  You said "without the batteries" so I guess you're looking to heat water efficiently using 120v current?

It depends on how much water you're talking about and how much you want to heat it.  A fixed-output heater will make water a lot hotter when the ambient temperature is 80F than it will when the ambient temperature is -15F.

I was hunting around a couple months ago for a temperature controller that I could use to heat some plastic up to just below its melting point, a target temperature of around 350F.  I didn't have the money to buy all the components needed but did learn some stuff.

fwiw, if I was going to heat water efficiently, I'd build my own inline water heater based on heating elements taken from hot-glue guns.  There are some hot-glue guns you can buy at walmart for about $10 each, one of these contains a single heating element that is physically cylindrical with an outside diameter of 5/16" and a length of about 1-3/4".  I'd use some brass fittings so the water could flow past a tube that contained one of these elements (with the element isolated from the water).  The ones I've played with were 40watts and heated to a temperature of around 365F.  That means you don't want to try using solder unless you want to risk the possibility of it disassembling itself on the job.

Where are you located ? .

The controller you envision, which is a ballast PWM controller to load the wind mill with just the necessary load to harvest the energy available at any moment require that you supply the power curve and voltage curve of the wind mill to see what would be the best PWM profile the wind mill needs to supply is latent available power to the heater ballasts.

The other answers, you have gotten, give you a different direction, based on ambient temperature which for sure will not load the wind mill properly.

If you have up to 5 KW, it may be best to have 3 loads, one that is PWM and 2 that sequentially inserted/removed and needed to maintain proper wind mill loading.

Your heating should be done into a "reservoir tank" and from there a secondary loop to heat what you need to keep at certain temperature, like the house ambient temperature.

Adding, as well, the MPPT capability to harvest the maximum available power and, as well, to protect the wind mill for over speeding at high wind regimes.

What are your electronic capabilities, specially, to assemble a circuit with several Integrated Circuits and trouble shoot it if necessary ?.

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