The threat of being killed by sharks is a very powerful catalyst for Pi; they keep him in the boat, under many circumstances when he'd rather be anywhere else. Knowing the sharks mean possible death forces Pi to become more inventive about ways to deal with his predicament. He has to find a way to cope with Richard Parker, for example, because he cannot risk dealing with the sharks, and Richard Parker is actually controllable in a way the sharks are not. So in many ways, the sharks represent outside forces (the dangers of his personal reality) that compel Pi to deal with what's actually happening on the boat. They are a boundary line past which he cannot cross, on a psychological level.
During the second evening in the lifeboat, when Pi has seen many sharks around the boat, the hyena goes crazy and attacks the zebra despite Richard Parker's presence. It bites and gnaws it so savagely that it digs into the animal. While Pi watches in silent horror, Orange Juice gives a roar of disapproval.
The hyena's break in behavior—attacking despite the tiger's presence, attacking more savagely than he needed to—foreshadows Pi's own descent into savagery. In addition, the sharks may symbolize what lies outside of Pi's immediate universe, or the safety of the life boat. What lies outside the lifeboat is savage death.
i dont think that there is any symbolim exept the grey shark and the grey tin of canned meat, both of which require Pi to step outside his comfort zone to deal with the stress and pain of them. Both of them involve animals and blood and therefore dying. pi is overcoming his paranoid fear of blood and chemistry and going through the major rite of passage suggested by martel in the book