Oki, it does work
you go to where, when you use the TAB button to cycle through all possible selections in the area, the river is one of them.
When you have it selected, go right up to it, it's like anything else, you have to be close to trade with it, so after selected, go up to it.
open menu, Trade, and put the empty skin in there, it should give you back the full skin of water.