You don't agree that some (probably most) people in the market for a water heater don't have incentive for the tsuris of figuring out all the details with getting a mining device running, connected to the internet, etc.?
I don't know. Depends how the device is designed. If it simply mines for the manufacturer and subsidizes the cost of the purchase, and if configuration is simple or nonexistent (perhaps a built-in cellular data chip with prepaid access), then I can see that working absolutely, and even possibly driving non-mining heaters off the market altogether. Many purchases are made without thinking past the up front cost.
I can't find a BTC miner with that wattage consumption for less than perhaps $20,000. I just briefly checked and only saw a 1250W one for $10,200.
I don't believe that is remotely applicable because you are paying for cutting edge power efficiency, for the case where that matters (high density deployments in data centers and mining farms), not the case where power is effectively free. If you can find them, you can buy older, less power-efficient miners for next to nothing. Underclock a bit so they run quietly and they make great room heaters!
It's also not a mass market product where chip prices can be driven down enormously. (How many microprocessors/microcontrollers would you imagine people own without even being aware of it? The number is quite high.)
Also, the mining component doesn't have to provide all the heat output, only some of it.