A 100 gallon pond in your geographical location will most likely freeze solid before the Winter is over. The pump is also likely to freeze as I doubt that it is over 500-750 gph. There are small pond heaters that are available that may work perfectly for a small pond like yours, but it will need to be functioning continuously during periods where icing over is possible. If the Lily is a hardy variety it should survive as long as the tuber does not freeze.
Another option is to relocate the entire pond to your garage where it will be more protected. The ambient heat from your house plus the use of a space heater may be enough to make it through the winter.
Pouring hot water into the pond is a sure way to kill the Goldfish. They can not handle abrupt changes in temperatures
"Everybody is ignorant, only on different subjects." - Will Rogers