Losing a bucket of water that has already been upgraded to gold must be frustrating. To help you in this situation, here are some simple suggestions:
1. Double-check the places that may have been overlooked or hidden. Often we can miss something by accident. Carefully inspect all rooms, boxes, and other storage areas.
2. Try talking to the villagers, especially Lewis. Sometimes they can acquire or take things unknowingly. Maybe Lewis took your bucket of water out of a misunderstanding or an urgent need.
3. Check to see if your bucket of water has accidentally been dropped or mixed with other items. Some places you might need to check are your bag, chest, or other storage space you have.
4. If all search efforts are fruitless, you may want to consider buying a new bucket of water. Check to see if there are other shops in Pelican Town that sell them, or you can try checking if there are any at the Gypsy Wagon.
5. Finally, if you are using mods in your game, make sure that this problem is not related to the mods. Incompatible mods can cause problems like this. Try disabling the mod and checking again.
Hopefully, these suggestions have helped you find your lost water bucket or provide an adequate alternative. Remain calm and thorough in your search, and hopefully this issue will be resolved soon.
Divine power flows in my veins, I am the chosen godkin who inherited the power of the gods, the "SEPUH" who reigns in the realm of Stardew Valley ( ̄⊿ ̄)╭