The sweetfish river is Beatrice's influence as it flows through history.
Kinzo's well-remembered hometown was washed away by it long ago.
And should anyone wish to find his gold, that person must swim down the river like the sweetfish, following its path and braving the cormorants - the servants of the witch who swallow up her words without question, while catching the sweetfish and bringing them to their master.
Eventually, that river leads to Rokkenjima, what is now the ancestral home of the Ushiromiya family - for a "village", when spelled with a "里", can also mean the home of one's parents.

The key to the Golden Land sleeps there - at the shore of the Witch's river, of which two humans will tell you.

Here is the river as it passes through Rokkenjima:
第一の晩に、鍵の選びし六人を生贄に捧げよ。   
第二の晩に、残されし者は寄り添う二人を引き裂け。
第三の晩に、残されし者は誉れ高き我が名を讃えよ。
第四の晩に、頭をえぐりて殺せ。         
第五の晩に、胸をえぐりて殺せ。         
第六の晩に、腹をえぐりて殺せ。         
第七の晩に、膝をえぐりて殺せ。         
第八の晩に、足をえぐりて殺せ。         
第九の晩に、魔女は蘇り、誰も生き残れはしない。 
第十の晩に、旅は終わり、黄金の郷に至るだろう。 
The Ten Twilights, a ceremony to revive the Witch. The kanji along the sides make the shore, and the straight line of kana along the middle show the stream itself. Upon which shore does the key reside? We know that the "two humans" will tell us of it, and we see them by the river - "the two who are close", who will be torn apart.
第二の晩に、残されし者は寄り添う二人を引き裂け。
Furthermore, the two humans tell us of the shore another way: if we take the first twilight as a labeling of columns, and the twilights' numbers as a labeling of rows...
第一の晩に、鍵の選びし六人を生贄に捧げよ。   
第二の晩に、残されし者は寄り添う二人を引き裂け。
第三の晩に、残されし者は誉れ高き我が名を讃えよ。
第四の晩に、頭をえぐりて殺せ。         
第五の晩に、胸をえぐりて殺せ。         
第六の晩に、腹をえぐりて殺せ。         
第七の晩に、膝をえぐりて殺せ。         
第八の晩に、足をえぐりて殺せ。         
第九の晩に、魔女は蘇り、誰も生き残れはしない。 
第十の晩に、旅は終わり、黄金の郷に至るだろう。 
Whichever method we use, the answer is the same. The two people tell us of this shore and this shore alone:
寄
If we read this kanji on its own, it speaks of "approaching", "meeting", and "coming near". But it is itself composed of a radical and another kanji. Let's tear it apart:
宀The "crown" radical
奇The kanji pronounced `ki`, and meaning "strange" or "mysterious"
The key is a strange crown, or something both strange and crowned, and is to be found on Rokkenjima when the witch's river intersects it, at the shore of her influence. Even to someone like me, who's only played EP1, it should be clear who is meant by this: none other than Battler's creepy younger cousin. She can even be read as herself being the "village" in which one should look for the key; her strange, un-Japanese name, 真里亞, has as its kanji first "true" or "authentic", then "village" as seen in the second line of the epitaph, and finally the ultra-obscure "亞" which means roughly "lower-ranked" or "follow". Together, these spell "true sub-village", the smaller 里 within the greater one. Of course, this means that Maria herself is not the key (or she would not still be the village). Rather, this makes her the one with whom the key resides.

My deduction from all of this is that Maria's strange crown, or at the very least another item in her possession, is the key to the Golden Land, the last clue necessary to find Kinzo's hidden gold. I haven't gotten any farther than that - I'm still at a disadvantage, having only played the first VN, and ten tons of gold can hardly be compressed into a miniature crown. The complete answer would require an examination of the crown itself, which can only be done by the characters themselves. But unless the Twilights are laid out differently from the left-aligned way that one would assume, or it's been jossed by Maria's backstory, I'm pretty darn sure of what I've got here.