

Any temperature below somewhere around 60F/15C is “deadly cold”, as in your survivability depends entirely in how well you are clothed as you will eventually die of hypothermia otherwise, the only variable being how long it takes. Kinda like how you can get a 3rd degree burn with 44C water, it just takes 6 hours.
-12C really isn’t all that cold - lowest temperature in northern Finland this winter so far has been -42,8C / -45F - but it is a temperature where you will need to pay some attention on how you dress for it. For me, it’s around (-10 to 15c depending on the wind) where I’ll put on long-johns in the morning and add a sweater instead of just having a t-shirt under my jacket.

Well, yes. Dumping high concentrations will instantly kill everything in the waterway, diluting them and doing it slowly means they can handle it and survived.
Heck, the ocean is full of salt, but if you started dumping high-concentrated brine off a beach you’d kill every animal and plant on sight, just as you would kill yourself drinking said brine. But it would be quite hard to argue that you can’t safely put salt in the ocean, or add some to your food, once it is diluted to a safe level.
The question is how much of something total can the ocean handle before it becomes a problem. And for many things the answer is, quite literally, that it is just a drop in the ocean.