A Chinese doctor recommended a gold, or silver or copper coin or nugget placed into water. This is not for good luck or avodah zara in any way. Does one have to tovel the coin or nugget since it touches water?
No, the gold is not being used as a kli seudah [as a vessel], rather as one of the ingredients of what you are consuming. Tevila is only when a metal kli is being used to eat other foods in some way.