"Water" is a common name in China. It was originally called "water" because of the fineness of silver. When the silver is insufficient, it should be replenished. This is called "slurry water". Then the document filled in is a memorandum. At present, this term has been widely used in original documents that flow like water.
In the past, all foreign exchange had to go by water, so it was commonly called memo, which extended to this (it sounds rational).
In the past, due to the underdeveloped information technology, foreign exchange transactions between China and other countries had to be transported by water, so it was called "water bill" and has continued to this day. Memo is the general term for foreign exchange sales receipts. It is the process of changing water in different places and different foreign currencies.