what percent of earth's surface is covered with water?
out of this, what is the persent of potable water?
out of this, what is the rate of water re-use?

First, according to WorldAtlas.com

http://www.worldatlas.com/aatlas/infopage/oceans.htm

the basic measurements of the Earth's surface are as follows:

"Surface Area of the Planet (510,066,000 sq km) 
Land Area on the Planet (148,647,000 sq km) 29.1% 
Ocean Area (335,258,000 sq km) 
Total Water Area (361,419,000 sq km) 70.9% 
Type of Water (97% salt), (3% fresh)"

So the answer to your first question is that 71% of the Earth's
surface is covered with water. Taking "potable" water to be fresh
water as opposed to salt water, 3% of that total is potable.
Obviously, much of that water is not, in fact, immediately potable,
but relatively inexpensive filtering and purification systems are
sufficient to treat most of it. Making salt water potable is still a
much more expensive undertaking.

I'm not sure exactly what you mean by the rate of water re-use. In
general, water naturally cycles through the environment with a lot of
different time-scale. Some can be trapped for thousands or even
millions of years in glaciers, ice-caps, underground reservoirs and
aquifers (if you're interested, you can read about so-called "fossil
water" at a number of sites; just do the Google search:

www.google.com/search?q="fossil water" ).

Most surface water is recycled on a much shorter time scale by
weather, river flow, ocean currents and biological processes. See, for
example, "Water Science for Schools" at the USGS website:

http://ga.water.usgs.gov/edu/followdrip.html

There are still a lot of different time scales at work. For example,
there are daily cycles as water condenses and falls as rain or dew and
re-evaporates, and there are yearly cycles as water accumulates in
mountain snow pack, then melts and flows back to the sea. Water is
also important to a great many chemical reactions, so some water is
converted to other molecules and other molecules are converted to
water. Given the huge volume of water that exists on the planet, even
though a lot of water may participate in such reactions, it probably
has only a minor influence on the global water cycles.

Then, of course, you could talk about human use, contamination and
recovery via wastewater treatment facilities. You might also want to
know something about the availability of potable drinking water around
the world and the problems of water availability relative to
population density. Natural processes don't always concentrate water
in excess quantities everywhere where people choose to live. This can
generate a lot of conflict and even war. See, for example the
chronology of water-related conflicts at WorldWater.org:

http://www.worldwater.org/conflict.htm

Other searches which will generate further information:

://www.google.com/search?q=area+water+world
://www.google.com/search?q=water+cycle
://www.google.com/search?q=water+supply+world

A fast and complete answer.

Regarding how much of the fresh water is actually usable, this
information from the World Health Organization's "Water and
Sanitation" webpage might be useful

http://www.who.int/water_sanitation_health/vector/water_resources.htm


Of all water on earth, 97.5% is salt water, and of the remaining 2.5%
fresh water, some 70% is frozen in the polar icecaps. The other 30% is
mostly present as soil moisture or lies in underground aquifers. In
the end, less than 1% of the world's fresh water (or about 0.007% of
all water on earth) is readily accessible for direct human uses. It is
found in lakes, rivers, reservoirs and in underground sources shallow
enough to be tapped at affordable cost. Evaporation and precipitation
make this water available on a sustainable basis.

Much of the approximately 110 000 cubic kilometers of precipitation
that falls on the continents each year evaporates back into the
atmosphere, or is absorbed by plants. About 42,700 cubic kilometers of
water that falls back on earth flows through the world's rivers.
Dividing the world's total river flow by its 1995 population gives an
average of 7300 cubic meters of water per person per year, a drop of
37% per person since 1970 because of the growing world population.

Fresh water resources are unevenly distributed: in terms of
precipitation, there is a range from almost no rainfall in deserts to
several meters per year in the most humid regions. Most of the flow is
in a limited number of rivers: the Amazon carries 16% of global
run-off, while the Congo-Zaire river basin carries one third of the
river flow in all of Africa. Arid and semi-arid zones of the world,
constituting 40% of the land mass, have only 2% of global run-off.
River flow, however, varies greatly over time and evaporation plays an
important role whether a country can be classified as humid or
semi-arid. The avergae annual run-off is determined by the population
size. Per capita availability of water is lowest in Asia, even though
it has the world's greatest river flow. In Australia/Oceania on the
other hand, the per capita run-off is high, in spite of the fact that
most of the continent is dry.

Experts have estimated the amount of the world's river flow that is
readily accessible for human use at about 9000 cubic kilometers a
year. They add another 3500 cubic kilometers of river flow that is
captured and stored by dams and reservoirs. Currently, humans are
using about half of the 12500 cubib kilometers available. This may
seem to leave a lot of room, but in reality water needs to be left to
sustain health ecosystems (in particular wetlands and their
biodiversity) , to permit fisheries, and to keep an adequate amount of
water available for recreation, navigation and hydro power generation.

Thanks!
A fast and complete answer.