I think your first problem is the shack is too close to the ice. What are using for skids? Minimum should be pressure treated 4x4s, or 4x6. Failing that you are far enuf away, what are the size of your holes in the floor?You can cut the bottoms off of 5 gallon pails and put it in the hole before drilling, will keep most of the water in the hole, as long as you can drill right throught now! Other than that, do what you said, shut it off or lift it slowly out or bring a shovel and a broom and clean up after, than stoke the furnace and dry the place out.