lake bottom?

Pawing the water furiously and futilely, I look up to see lake bottom where the sky should be.

This is a sentence after the author has dropped into the water, drowning.
What is "lake bottom"? I have thought it should be the deepest part of the lake, which is far below the surface. But I can't understand the sentence.
After dropped into the water, even if he has a floatingvest, why can he look up and see the deepest part of the lake? Even if he's upside down, why the sky is under the lake at the deepest part of the lake?

So what is "lake bottom"? I'm confused. Thank you for help.

http://www.latimes.com/features/outdoors/la-os-tahoe28jun28,0,7390858.story?coll=la-home-outdoors

"Don't you drown! Just don't you drown," I think as icy Lake Tahoe surges into my lungs like a shock-freeze blast of inhaled Slurpee.

Pawing the water furiously and futilely, I look up to see lake bottom where the sky should be. A moment before, the kayak paddle rested on my lap as I closed my eyes, seduced by snowy peaks and the butterscotch scent of Jeffrey pines, glad to be free of whitecaps blasting out of Emerald Bay.

Must get out!" But how? The advice of my paddling partner and guide flashes through my mind: Put your legs together, pull them up, kick to break the skirt seal of the kayak. I fight panic and perform each step, swoosh to the surface, gasp and live.

Thank you.
And you are so considerate, JTT.

Some folks would disagree, vehemently in fact, but thank you, T and you too, Joe.

JTT wrote:

Some folks would disagree, vehemently in fact, but thank you, T and you too, Joe.

Don't count me in that group, JTT. Your countercultural viewpoints are much needed yang in a forum full of yin. Good catch on the context. It serves to remind posters to include any important context (actual, not interpreted) when inquiring about a word, phrase, or sentence.

Looking at the context now, I'm quite surprised that Translat got past the "shock-freeze blast of inhaled Slurpee" line. Translat, do you know what a Slurpee is?