Q. Is it better to say tashlich by a flowing river if one has a choice or can it be said lekatchila by a lake or the sea?
A. Maharil mentions that the source for the minhag of tashlich is the midrash (Yalkut Shimoni, Vayeira 99), that describes how Avrohom and Yitzchak were walking on the way to akeida and were obstructed by Satan who turned himself into a river. They continued to immerse in the water up to their mouths and prayed to Hashem to allow them to complete their mitzva. Since that day was Rosh Hashana, we use the body of water, which conjures up memory of the merit of the forefathers, as an appropriate place to ask that our sins be “thrown into the depths of the sea.” (Micha 7: 19).
Although the sea is mentioned in that posuk, a river as a sign of blessing because of how it flows or as also being the historic place for the coronation of kings (see Yechave Da’as I: 56).
Remah (O.H, 583: 2) mentions going to a river, and Divrei Zikoron quotes similar wording from the Sheloh.
However, many Poskim mention sea, lake or river indistinctly.
Poskim do mention to go to a place where fish are present.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit’a