About sharks and how they find fish
Sharks sense their prey with the normal senses, they see, hear and smell them.

They have a remarkable sensitive sense of smelling, which enables them to sense highly diluted traces of prey. They can also use their smelling to determine the direction where a certain smell comes from. This is achieved by the timing in which the senses arrive in different parts of the sharks nose. See references 1 and 2 for more details.

While sharks have eyes which are similar to that of vertebrates, they have an additional reflective tissue called "tapetum lucidum" behind their retina, which reflects the light back and therefore raises the sensitivity of the eye. It seems that a lot of shark species are either colorblind (because they only have rod cells but no cones) or they had only one kind of cone cells which allows them only to see one color. For sharks it is obviously more important to detect objects than to see colors. See references 3 and 4 for more details.

The sense of hearing of sharks is also highly sensitive and is able to detect sounds from far away. See also references 5 and 6 for more details.

The most interesting sense of sharks is their electrosensory organ. This is also called Ampullae of Lorenzini which is a number of sensory cells along the head and which are able to detect the weak electric fields that exist around living animals. This sense only works on a very short distance. See references 7 and 8 for more details.

Finally sharks have the so-called lateral lines (which are present in most fish), which is an organ which detects movements and vibrations in the water around the shark. See the Wikipedia page for more information.

And for the sake of completeness (already quite nicely answered by @Susan in the comments), sharks do get cancer. All cures which are made from shark and claim to be able to treat cancer are pseudoscientific nonsense. This also endangers many shark species as they get radically overfished. See references 9 and 10 for more details.



References:


Smell and Taste
The Function of Bilateral Odor Arrival Time Differences in Olfactory
Orientation of Sharks
Microspectrophotometric evidence for cone monochromacy in sharks.
Vision and a Carpet of Light
Hearing and Vibration Detection
Functional anatomy of the auditory system in sharks and rays
The Shark's Electric Sense
Electroreception
Mythbusting 101: Sharks will cure cancer
Shark cartilage, cancer and the growing threat of pseudoscience.