"Tashlich is preferably recited alongside a body of water containing fish, to remind us that just as fish are protected by the water in which they live, we pray to be protected by G-d. Also, just as fish swim freely and can suddenly be caught in a net, so too we can just as helplessly fall into the net of sin. And even as the eyes of fish are always open, so do we pray that G-d too will keep vigilant watch over his people."
(from the Orthodox Union website)Interestingly, the same webpage adds this reminder of the old days:
"Historically, Jews would sometimes be instructed by their own community leaders to seek out bodies of water not near the center of town, so as not to be seen during Tashlich by their non-Jewish neighbors, for fear of a 'poisoning the wells' accusation. This was certainly true if actual food was thrown into the water, but even if not, they would be accused of mumbling curses and poisoning the wells by witchcraft."
We have all come a long way since then [I hope].