I've seen that there are watercooling solutions for S19's, but they draw a bit more power than I want to give it based on my estimates.
The problem here would be your power range, so you will need to disable 2 boards and keep only one, with those S19s removing two hashboards leave you at slightly below 900w, with whatsminers set on low you can get a single board down to 800w range.
Of course this makes the cost per th a lot higher, and on average you are going to get 30-35th out of those new gen miners, so anywhere between 1.7$ to 2$ a day.
You can check my miners sale topic for price refferences to see if the project is worth it, also water blocks cost between 100-200$ for the new gen miners like M30s and S19s.
The downside of your project is the limit of power, if you could do up to 2000w it would make a lot more sense.
Yes, I'm aware of that. The S19's are just the first "somewhat suitable" devices I found, but as you say, they are above the power range I think would be reasonable. I can run the numbers again and maybe I could go with 2000W (or disable some boards as you say), but I was wondering if there are perhaps other devices that are lower-powered out of the box.
However, as stated in the OP, the first priority is using available electricity to produce heat (not the mining itself), so in any case, I would only run the miners intermittently. And if I run them with 2000W, it means that the water I need heated is done sooner, so it would run for shorter periods of time. The total energy usage and thus the total hash produced over some time frame would be fixed anyway; so it might not be worth it anyway.
I will need to look closely at the costs of course. But disabling some of the boards sounds like at least one first potential option to consider, thanks!