Wayne, interesting you recommend bottom heater. Around here, bottom heaters are made to sit right on the bottom of a metal tank, and usually say cannot be used on anything other than metal. In other words, it would melt plastic or rubber. But, what you use must have some type of frame to keep it off of the liner. I would be very careful to have it right down on the liner, as there may be a fold in the liner right where the heater is, and you sure would not want to melt your liner, or cause a short in the heater. That's why most use the floating heaters. It's not a matter of heating all of the water (which you didn't say anyhow), but rather keeping an opening in the ice. You don't have to keep all the ice off, it's actually best to leave most of the ice intact, just melt a hole and keep an opening for the gases to be released. The air works well with the heater, or a pump pushing water into the same area of the heater. Either one will move the water, keep the area open.
Yes, if you get an 8 hour outage, you will have to reopen the hole in dead of winter, but a floating heater will do the trick. However, on my floater, it specifically says to not let it freeze into the water. I think the reason for that is that the water may freeze and expand to over the top of the heater, maybe that shouldn't happen, or could burst the casing it's in. On my pond, I only used the heater to melt through the ice, then turn the pump on. Pump stays on the bottom, never freezes down that low. BUT, I can't turn on the pump if there is thick ice, as it takes too long to work it's way through the ice. Another safety caution if you put the heater on top of the ice, WATCH IT! Mine started melting, and then started spinning, and pretty soon the heat rods were heating right through the electric cord! I caught it in time, but it could have shorted it out pronto. I'm tempted to leave it in the pond this year, just plug in when needed. Anyone else do that, let the ice freeze around the heater? Does it work well to plug it in to melt it out again then?