Philip R. Headings on "fish" and "fisher" symbolism:
In the vegetative rites discussed in both, [Frazer's The Golden Bough and Weston's From Ritual to Romance] the figure of the Year-god was thrown into the waters of the Nile (or some other body of water) and later "fished out" (resurrected), symbolizing the rebirth of the life principle in the spring. This ritual also came to be associated with the religious initiation patterns to which primitive people seem to give much more open recognition than do modern civilized societies. The Grail legends, according to Miss Weston, are derived from those vegetative rites, and it is the Fisher King on whom the health and fertility of the land and people are dependent in these legends. The Fisher King is sick, having been maimed (usually a sexual wound); and, because he is sick, his lands are waste and barren, just as in Oedipus Rex (as Tiresias knew) the plague upon Thebes was due to the crimes of Oedipus against the procreative cycles. Only when the Fisher King is healed through the appearing of a pure fool who asks the proper questions can the land again become fertile.
The relevance of this to the Christian-scheme is discussed by Miss Weston; it is summarized as follows by C.S. Fraser: "The Christian interpretation of this traditional myth is the highest one: the sacrificed king is Christ, as God Incarnate, and the barren land which has to be reclaimed to fertility is the human heart, full of selfishness and lust, choked with the tares of sin."
The inevitability of the "fish" and "fisher" religious symbolism is seen by reflecting on the high degree to which the early peoples were dependent on rivers and seas, the fecundity and vitality of fishes, and the mysterious "grace" which brings the fish to the fisherman. Thus Buddha, for one example, was represented as sitting on the bank of the ocean of Samsara, casting for the fish of Truth to draw it to the light of salvation; and Christ, for another, offered to make his disciples "fishers of men."
See also Weston on Buddha as the Fisherman.