T Farmer wrote

I have a 50 gal plastic barrel that I fill 2/3 with water, to
use in case of a water outage. I would like to keep to from
freezing this winter (it's in my garage, which isn't insulated).

Can't you keep it in the basement or the house, its not that big.

I bought a vat of insulation to wrap around the barrel
but someone told me that wouldn't keep it from freezing.

He's right.

So I just bought some heat tape (I think it's called),

Yes.

but the salesperson told me it might
melt the plastic because it get pretty hot.

You can get heat belts and pads that are used for plastic
beer brewing barrels which dont melt the barrel.

Or use an electric blanket.

If I wrapped the insulation around the the barrel,
then wrapped the heat tape around the insulation
would that work to keep the water from freezing

Yes it should do.

or would it create a fire hazard?

Depends on the insulation. Rockwool or glass fiber
would be fine, polyfoam would be a fire hazard.

Worth adding a thermostat so it only heats when
the garage is below freezing to save on electricity.