If you need to warm it up depends on the depth - at the bottom the ice is not much colder than 0°C due to geothermal flux. (That's why the lake is there too.) S

If you need to warm it up depends on the depth - at the bottom the ice is not much colder than 0°C due to geothermal flux. (That's why the lake is there too.) So if, say, you only used the hot water method for the lowest part, the amount of heat for warming can easily be neglected.