When a shark "smells" blood in the water, what is it "smelling?" -
what is the compound, chemical or whatever.

It appears that amino acids can be recognized by sharks in very small
concentrations:

"Sharks can recognize most amino acids, the basic component of
proteins, even in such small concentrations of 10-10 molar (a "one
molar" solution is equivalent to the molecular weight of a substance
in grams dissolved in one liter of water). Permit me to express this
astronishing sensory perception in a more understandable manner.
First, it means that sharks smell about 10,000 times better than
humans. Second, to reach this concentration about 30 mg (measuring
half a pinch, editor's comment) of an amino acid would have to be
dissolved in a large swimming pool filled with saltwater! Third, such
a swimming pool would have to have a volume of 2,600 cubic meters and
this amount of saltwater would contain 3.5% of cooking salt and other
ions, in other words, an approximate total of about 92 tons of salt.
It is very hard to imagine that a shark can still perceive these few
milligrams of amino acid when mixed with tons of salt. Fourth, sharks
can even sense the presence of certain amino acids better than others.
For example, they smell the amino acid Serin in concentrations of only
10-14 molar!"

http://www.sharkinfo.ch/SI3_99e/noses.html

Interesting. If amino acids are the ticket, what amino acid(s) make(s)
blood so special to sharks?

Here's another mention of the amino acid serin (serine) as a chemical
stimulus to which sharks have great sensitivity:

"The olfactory sensitivity of sharks in general is nearly legendary,
fostered by countless wide-eyed stories of these predators following a
trail of blood a quarter-mile (four-tenths of a kilometre) or more to
its source. Laboratory tests of shark olfactory acuity have revealed
that even these anecdotal tales pale in comparison to carefully
measured reality. Experiments on isolated olfactory lamellae of
certain skates (family Rajidae) have revealed astonishingly low
threshholds to chemical stimuli -- responding to concentrations as low
as 10-14 moles per litre of water for the amino acid serine (or about
1 molecule of serine in 1015 molecules of water). In terms of relative
volume, this is comparable to detecting a golf ball in Loch Ness."

http://www.elasmo-research.org/education/white_shark/smell.htm