How much of the ice on earth is floating?
A first-order estimate is about 5%.

This is a trickier question than it first appears because of ice shelves. Quoting from Kusky (2014):


  Ice shelves form where ice sheets move over ocean waters and form a thick sheet of ice floating on the water and attached to the land on one, two, or three sides. Their seaward sides are typically marked by a steep cliff up to 1500 ft (500 m) high, where many glaciers calve off from forming icebergs.


So they are floating, and they can be up to 2000 m thick, according to antacticglaciers.org. So they definitely make a substantial contribution to the world's sea ice fraction.

Continuing with Kusky:


  Ice shelves are found in Antarctica, Greenland, and along the polar seacoasts of the Canadian Arctic Islands. The largest ice shelves are contained in Antarctica, which contains 91% of the world's glacial ice, around 7% of which is contained in ice shelves. These ice shelves cover 50% of the coast of Antarctica, forming an area 1/10th the size of the continent.


If you don't take ice shelves into account, and only think about pack ice, you end up with about 0%, because pack ice is generally thinner than about 4 m, and so makes up a tiny fraction of global ice volume, in spite of its extensive area.

But if you do account for ice shelves, especially the southern ones, the answer must be much greater. I have not done the research to find the numbers, but the text I quoted suggests something less than 7%, so I'm going with 5% as a first-order, round-number guess.

Reference

Kusky, T M (2014), Encyclopedia of Earth Science, New York Academy of Sciences, 529 pp. In Google Books.