I've used saltwater to treat cold-sores and sore throats. I didn't use very
hot water, but very warm. I would dissolve the salt until it created a
precipitate then would gargle and then swallow some to coat my throat.

I have also had some success with a strong tea made from Calendula flowers
and used as a gargle. It tastes bad but seems to work (for me at least.)

Another option I've tried is a mouthwash containing Melaleuca Oil. This
essential oil has potent anti-septic properties. I've swallowed a tiny
amount over the course of a couple of days and it proved effective (again,
for me at least.)