David Blaize and the Blue Door by E.F. Benson

It was neither cold nor wet below the glass roof of the lake, for, as David already knew, when you are completely in the water, from your head to your heels, you never think of saying ‘Oh, how wet it is! ' and it is only when a piece of you is wet, like when you are washing your hands, or a snowball goes down your neck, that you think of wetness. Certainly also it was not cold, because there were so many red-leaf fires burning. Up overhead the moon shone very brightly through what David knew was ice to the ordinary world, but which it was much more correct to call fish-glass, and it made the most lovely lights in it, just as if all the diamond tiaras and emerald and ruby necklaces had been mashed up in the fish-glass.

‘That's something to know,' he said to himself. ‘When there's fish-glass on the lake, I shall make a hole in it and get underneath. What nonsense grown-up people talk! They all say it's dangerous to get under the ice-fish-glass, but it was the only safe thing to do. I suppose I'd better call on some fish and thank them for rescuing me.'