I am using Blue Tilapia, claim that they can handle colder weather if needed down to 45F
That's usually no lower than 50F and it all depends on how deep the water is and how long the temp stays below 50F. I raised approx. 300 Blues in my swimming pool last year and temps dipped into the 20s numerous nights but were always back up into the 40s and 50s during the day...never lost any but the water is 9' deep.

Yes submersable heaters do work but they are tircky because you can set them high enough to keep the water warm during the low temps at night but they tend to overheat the water when the temp comes back up during the day. But there again, it's according to how much water volume you are heating and how deep it is. Close monitoring is the only way to not cook your fish.