There is a mystery to the fish. Much of humanity relies upon it, but for most of history we knew so little about the world it came from. Emissary from a strange and alien place, inaccessible to us. Perhaps this played a part in the choice of fish as an early symbol for Christ, another emissary from the unknown. More likely fish as Christ came about because of the fish's preexisting symbolic association with life, shorthand for the life sustaining bounty of the seas.
And Christ as fish, life giver and messenger, makes a direct contrast to the Lord of the Old Testament who most of all resembles that most terrible of his supposed creations, the Leviathan. Master of the alien world. Remote, powerful, destructive and mysterious. Rarely seen, most often recognized by the signs of its passing. The cresting wave signaling the colossal thing passing beneath. A great wind that rends mountains signalling the thing that swims unseen behind the material world.
In the Book of Job the Lord describes the terrible power of the Leviathan to cow Job into compliance, but he doesn't stop with the terror of the seas. He also boats of another of his creations: The Behemoth. Leviathan's land locked counterpart, a beast of such enormity it drinks rivers. And if we take Behemoth as another symbol of the Lord then we see even more why Christ, the fish to the Lord's Leviathan, came to Earth as a man.
According to Jewish folklore there is one other primal beast to rival Leviathan and Behemoth. It's name is Ziz and it is the terror of the skies. A bird of such enormity its wings block out the sky. Now we recall that the dove is yet another symbol for Christ. Fish, man, and dove versus Leviathan, Behemoth, and Ziz. Two competing views of divinity and therefore the nature of the universe. A place of life and hope or a place of terror and death.