The necessary elaborateness of a waterborne attack makes it non-viable.

If you've ever had a fish tank you know how many chemical reactions go on in water.

The best you could hope for is something like e-coli.

Now, water contains more oxygen the colder it gets (another thing that affects chemical reactions, making chemical attacks a pain using water vector).

For biological water borne attacks, that's great. Organisms need the extra oxygen.

That is me doing my best to support your question.

I still don't think it's a concern.

Lastly, pipes burst all the time in the winter. That causes boil orders to go out for anyone potentially affected in a wider radius than you'd think.