They can smell it. Water works just as well as air at conveying smells, provided you are built to receive them. Sharks have been shown to detect odours at distances of just under 100 metres.

It is blood, or more accurately, the chemicals in the blood that they are "smelling". Sharks have an acute sense of smell. They are well-known for their ability to detect minute quantities of substances such as blood in the water. Sharks can detect a concentration as low as one part per billion of some chemicals, such as certain amino acids. A shark's sense of smell functions up to hundreds of meters away from a source. http://www.seaworld.org/animal-info/info-books/sharks-&-rays/senses.htm Q: What is mechanism that allows a shark to smell blood at great distances in the water? A: Actually, sharks cannot smell anything from a great distance. What they can do phenomenally well is sense extremely minute quantities of dissolved chemical 1,300 feet (400 metres) or more from their source. But, before it can be detected as a 'smell', a tiny sample of that chemical must come into direct contact with specialized chemically-sensitive tissue known as 'olfactory epithelium'. <snipped> The sensitivity of sharks to water-borne chemicals is often so acute, they can actually distinguish which nare — the left or the right — is receiving the stronger whiff! Thus, by comparing the strength of a chemical signal between its two nares, a shark can quickly home in on the source of an attractive odor. In hammerhead sharks, the nares are located near the tips of the hammer and may be separated by a metre or more. Special grooves along the leading edge of the hammer help channel scent-bearing water to the nares. As a result, in warm waters, hammerheads are often among the first sharks to arrive where fish scraps have been thrown overboard. And lastly, certain chemicals — such as volatile oils — often indicate a rich food source and can be rapidly carried long distances by wind. A recent Russian study has demonstrated that the Schneiderian folds of at least one shark species, the Oceanic Whitetip (Carcharhinus longimanus), are spaced in such away that they can capture air bubbles at the surface. The ability to sample the air chemically may enable these sharks to locate the source of an attractive odor more quickly than species which cannot. This would give the Oceanic Whitetip a major competitive advantage over other sharks in the otherwise featureless expanse of the open sea. http://www.elasmo-research.org/education/topics/s_nose.htm

only a pool shark would because pools usually have a bright reflective bottom and clean water.

Sharks can smell blood from many miles away. I am fascinated by sharks and find this to be one of the creepiest facts about them. Talk about survival skills!

nothing to do with the colour of blood. They can detect it in parts per million