In my city there is the sea and even a river. The local tradition is, however, to do Tashlich ceremony in a garden, before a pit that contains water (water well). Is this custom correct? Is going to the sea or the river better?

I think that the concepts of "Al Yifrosh Min Hatzibur" and "Be'rov Am Hadrat Melech" outweigh going to a place where there is fish. This might not be correct according to Kabalah, but I don't think we have to take this into consideration in this situation. Rabbi A. Rosenthal