Eavestrough heating cable

I've seen a few of you using Eavestrough deicing cable to warm/de-ice your
pond. I gather this stuff does not get warm enough to melt the liner? Does
it leak current into the water?

I did some measuring today and I only seem to have 24" of depth, so I'm a
little concerned that the bubbler won't do it. I could swear I had 30" (must
be old age ), but I'm not sure I was hitting bottom either. The netting
was obscuring my view. The plan is to set up a temperature sensor about 8"
off the bottom and use the deicing cable to maintain a 40 degree temp. I
don't really want to heat the water, just keep it warm enough to prevent
fishsicles. Do you think 300 watts will be enough if I cover the pond?